Senior school nurse Jane Young fast-forwards the sex-education video to the "nitty gritty", 40-second segment most likely to unsettle the parents of 10-year-old primary pupils at Sketchley Hill primary school, Hinckley in Leicestershire. "You can feel some parents squirm at this point," says fellow school nurse Jane Matthews.

So far, the Living and Growing video, made by Channel 4, has featured children talking about change. The message is that development, from new likes and dislikes to puberty's remodelling of bodies, is natural and absolutely nothing to worry about. Diagrams have shown the differences in girls' and boys' bodies and the way the penis can be used for both peeing and making babies. Apparently, it's the peeing part that always makes the children laugh. What makes me smile is the inclusion of the clitoris in the body diagrams, along with the information that sometimes it, like the penis, can get hard and "feel nice". Maybe this generation of girls won't be still looking for theirs in their 20s, like I was.

Eventually, we reach the critical point - and a cartoon man and woman have sexual intercourse. I'm struck that the jaunty little video makes it clear that the man and woman are having rather a lot of fun and that it puts the woman on top. I'm also struck by the clarity of the information, for clarity was always something sexual education lacked. In the 1970s, my youngest sister was around the same age as the children who see this video when she informed me and her other sister that school had finally taught her what sex was. "It is when a man and a woman get into bed together and strange things happen in the night," she said.

There are those who would rather that this generation of children continued to retain my little sister's "innocence" - and well beyond the age of 10. To inform children and teenagers about sex and contraception, the rightwing press has thundered in the past few weeks, is to encourage early sex, and pay the price in high levels of teen pregnancy, abortion and sexually transmitted infections (STIs). What schools should teach, if they teach anything, is abstinence until maturity - and, ideally, marriage.

Teaching unions, children's charities, FPA (formerly the Family Planning Association) and Brook, which provides sexual health advice to under 25s, all argue the opposite. We still have the highest teen-pregnancy rate in western Europe because kids know too little, not too much. FPA and Brook this week called for sex and relationship education (SRE) to become compulsory in primary and secondary schools.

That demand is being backed by young people themselves through the UK youth parliament. In a recent survey by the parliament of 22,000 school students, 40% said their SRE was poor or very poor, while 33% said it was no better than average. In addition, 55% of 12- to 15-year-olds said they had not been taught how to use a condom, and neither had 57% of girls aged between 16 and 17 - despite a government recommendation in 2000 that this be taught. More than half had never been told where to find their local sexual health clinic.

The sex-ed conservatives have not changed their view since I raced home from primary six more than three decades ago with an invitation for my parents to the premiere of my school's first sex education film. The "dirty film" was the talk of the Co-op for months and my Catholic granny thought it was a disgrace. My parents saw the film and allowed me to see it too, but not everyone's parents did. There was plenty of political and religious opposition then to "innocents" seeing a film destined to swell the ranks of the "gymslip mum".

In recent weeks, sex-ed conservatives have fumed at new figures that show a 10% increase in abortions for girls under 16, and a 21% rise for under-14s, despite the government's efforts to tackle the UK's high teen-pregnancy rate through sex education and easier access to sexual health services and contraception. Though teen pregnancies have fallen in the UK by 13% since 2000, and are at the lowest level for 20 years, the government is nowhere near meeting its target of a 50% reduction by 2011.

Conservatives have also been provoked by a new study from the Sex Education Forum (SEF) which claims that almost one in three secondary schools now provides on-site sexual health services. According to some newspapers, teachers are "dishing out" condoms and morning-after pills to children as young as 11 - and even encouraging them to have abortions - without their parents knowing. As a head of steam builds for a radical overhaul of sex education, just exactly what are our children learning about sex and relationships in school?

The first thing to understand is that the quantity and quality depends entirely on which school a child attends. Primary schools are currently under no legal obligation to provide SRE beyond the science curriculum's coverage of the biological basics - the "plumbing", as Sarah Smart, manager of the Personal, Social and Health Education (PSHE) Association, refers to it - and there is therefore a great variety of practice.

While a few rare schools choose to provide nothing, or almost nothing, the more common practice is to offer SRE for the first time in primary year 6, just before children go on to secondary school. But, even then, that SRE can amount to just one visit by the school nurse to talk about periods. Smart makes it clear that such practice hardly deserves the title sex and relationship education. "What kind of message does it give children about sex when it is the nurse that comes in to take the only lesson?" says Smart.

The most common experience for primary children is to have SRE - over at least a few sessions - in both year 5 and 6. "But that isn't satisfactory either," says Smart. "It still comes from nowhere. It doesn't build on anything. It's not the kind of natural, gradual, incremental, needs-led learning about sex and relationships that we want to see. It's too little, too late."

Some parents fear that compulsory SRE would lead to five-year-olds watching explicit videos of couples having sexual intercourse. In fact, they would simply be introduced to ideas such as development - asked to think how they have changed since they were toddlers - and encouraged to consider why they like their friend or what makes them happy or sad.

In many secondaries, SRE is delivered as part of the PSHE curriculum, though it is also not compulsory and is often squeezed out by other subjects. In the worst situations, a poor primary experience is followed by a secondary school experience in which SRE amounts to a one-off annual sexual health roadshow where outside organisations come in to talk about STIs or contraception. Relationships and other factors that impact on early sexual activity - such as feelings, peer pressure, confidence and self-esteem - don't figure. "These events can be useful but not in isolation," says Smart. "They don't work as a bolt-on. And there's no opportunity for follow-up. We wouldn't teach any other subject like this."

Sketchley Hill would be an example of good practice. It goes beyond what many primaries provide. When headteacher Susan Lees arrived at the school in 1996, sex education was taught only once, and very briefly, in the final year. "The year 6 girls did periods while the boys did road safety," she says, arching her eyebrows.

Lees thought the periods/road safety combo inadequate and so did Jane Young, the school nurse. First, they reached back to year 5 (nine- and 10-year-olds) with information once thought only suitable for year 6. Then, a few years ago, they introduced basics, such as the proper names for sexual parts of the body, to year 4 (eight- and nine-year-olds). Young, now promoted, has spread the approach to surrounding schools.

What some commentators fail to appreciate, says Lees, is that puberty is arriving earlier for girls. "We have girls now having their periods when they are just nine," she says. "I even had a girl who joined us from another school who had her first period in our school toilet at eight. She didn't know anything and she thought she was dying. That just isn't right."

Earlier maturation isn't the only reason Lees supports SRE in primary schools. Like the UK Youth Parliament, she believes children have a right to know about the emotional and physical changes of puberty, both their own and those of the opposite sex. "So boys now hear about girls' periods and girls hear about boys' wet dreams," she says, very matter of fact.

Lees aims to turn out self-confident, well-informed children through a programme of "age-appropriate" learning that teaches children about sex and relationships. So, those in year 5 see the Channel 4 "changes" video and learn how babies are made. But they also talk about self-esteem and self-confidence, and friendship and love, and how they should never feel pressured into doing things that don't feel right.

Year 6 focuses on hygiene and puberty since, by this stage, children are already having to cope with spots and hair suddenly sprouting up in the weirdest places. Lees says year 6 also learns a bit about bisexual and gay relationships, but insists that the kids - au fait with the gay plotlines of EastEnders and Coronation Street - take it all in their stride.

Young, who works in local primaries and secondaries, believes passionately that better sex education will further reduce teen pregnancies in Leicestershire. She is irked by claims that teachers and health professionals are giving contraceptives to 11-year-olds and encouraging early sexual activity. She says the comments are wide of the mark. Professionals, she argues, simply face up to a reality that many parents prefer to ignore: children are growing up in a society in which they are bombarded by sexual images, and that puts pressure on them.

"We actually challenge the attitudes that young people have about early sex," she says. "We talk about delay. When I was at school there was a period of finding out before you actually had sex. Now everything has accelerated and young people dive right into relationships they are not able to manage."

This theme is picked up by Cambridge psychologist Dr Terri Apter, who says that the strict line between childhood and adulthood has broken down and children are more aware of adult problems and sexual feelings than they once were. She says it is "too simplistic" to say that childhood was once innocent and that now it isn't, but that today children and young people tend to see "sex as pervasive and pervasively significant". In short, they are seduced into thinking it is more important that it is.

Apter doesn't think it is necessarily bad that the line between adulthood and childhood has been breached - she thinks communication between adults and children is sometimes better - but she says we have to work hard to help children understand the complexities of adult life and sex and relationships if they are to be protected.

In their efforts to help and prepare their children, Lees and her school nurses have also managed to keep Sketchley's governors and parents on board, even those who initially "squirmed" at the copulating cartoon couple. Schools are legally obliged to inform parents of SRE programmes, and when the "changes" video was first introduced in year 5, parents were invited to school for a screening. Sketchley Hill serves a predominantly middle-class, suburban population and on parents' night nearly every parent pitches up. The first showing of the video got the same big turnout, but now only half of the invited parents turn up to see it. Lees sees that as evidence of acceptance.

Matthews and Young say that if they have learned anything in their dealings with parents, it is to avoid using the word sex and to present SRE within the wider PSHE framework. They say that many parents didn't have any sex education and find the word "sex" in relation to their children terrifying.

Philippa Campbell, a teacher and parent at Sketchley Hill, says that she does have friends who think that sex education is inappropriate at primary level, and that her husband was worried the video would rob their son and daughter of their childhood. "I was able to reassure him that it wouldn't do that," she says. "As a mum, however, I don't want my children to learn what sex is for the first time at school. Parents can talk about this to children before it comes up at school." Nurses Young and Matthews, however, say it is astounding how many parents do not discuss sex with their kids.

Lees is happy to chat, but some secondary-school headteachers, it seems, would rather have their eyes plucked out than have a journalist in for a chat about SRE. The SEF study suggests that almost 1,000 secondary schools in England and Wales are providing SRE, as well as on-site sexual health services, but only one of the dozen high schools I approach agrees to a visit. Some heads claim they are too busy and others simply don't return repeated calls. A few PSHE teachers confide privately that their heads are terrified of speaking out.

Some PSHEs describe the delivery of SRE as like walking a tightrope. "We don't have parents evening on SRE," says one, from another Midlands school. "Most of the parents are happy with what we do and the kids appreciate it - they want to know. However, we have a large Muslim intake and a minority of Muslim fathers are a problem. They say sex education will make their daughters promiscuous. So we don't make a big thing of telling what we do or of consulting. So far, that's worked for us, but it's delicate. There's always that fear that the minority will blow something out of proportion. Your other worst nightmare is being targeted by the Daily Mail and something being splashed all over the paper, out of context."

Eddie de Middelaer, head of Lutterworth College in Leicestershire, could write a PhD on that kind of coverage. In April last year, De Middelaer and his family were picketed at home by anti-abortion activists after the News of the World ran a story claiming that Lutterworth prescribed more morning-after pills to pupils than any other mixed secondary. Under the headline "St Sinians", the newspaper revealed that between 2003 and 2007, Lutterworth issued 345 morning-after pills - 86 a year - to girls "without their parents knowing".

The school fought back, arguing that health services were decided by the school governing body in consultation with parents, teachers, pupils and the primary care trust. It also pointed out it was one of the largest comprehensive schools in the country, with almost 2,000 students aged 14 to 19 and often 700 girls over 16 on the roll - making 86 pills a year a rather less dramatic figure.

Given the reluctance of other high schools to talk, Highlands - a mixed 11-18 comprehensive in Enfield, north London - is a breath of fresh air. The school's website flags up this summer's parent-teacher cricket match, complete with Pimm's stall, but it also lays out the school SRE policy, and the contact details for local sexual health services. They include 4YP (For Young People), a teen pregnancy organisation that sends out a "hip, interactive sexual health advice bus" across Enfield and Haringey to reach young people who might otherwise have unprotected sex. Highlands serves a largely leafy, middle-class area, but Enfield borough is battling one of the highest teen pregnancy rates in London.

At Highlands, 12- to 14-year-olds learn more about puberty, reproduction and relationship skills. Contraception (complete with those controversial condom demonstrations), STIs and "family life, parenting and marriage" are taught from 14 to 16. Like Sketchley Hill, Highlands goes beyond both its legal obligations and what other schools provide.

Some argue that it's obscene to teach pupils below the age of consent about contraception. But Alice Smith, Highlands' PSHE teacher, says around 14 is the ideal age to deliver such lessons - just before the pupils get blase and start insisting they already know it all. She also says that she wouldn't take a chance on pupils waiting until 16 to have sex.

Between ages 12 and 16, classes also discuss homophobia and sexual orientation. Smith has also started a club for pupils called The So What (Sexual Orientation Whatever) Squad. You don't have to be gay to join - you may just support its stance against homophobic bullying - but the club offers support to students who are, or think they might be. Smith thinks there is only one other club like it in the country, but cutting-edge initiatives such as this dismay those with fond memories of Clause 28, as well as parents who believe that merely discussing homosexuality will "turn" their child gay.

So far, no parents have complained about the So What club. Indeed, parental concerns about the wider SRE policy are also rarely expressed, and only "a couple of times a year" are children withdrawn from SRE lessons, which is in line, despite all the sound and fury of late, with the very low national rate of withdrawal.

Smith says she does have to reassure some parents that she isn't promoting early sexual activity, though she says parents are naive if they think teenagers need her encouragement to be interested in sex. At this age, she says, it can be an obsession. "I actually try to get the students to look critically at the way they are bombarded with sexual images," she says. "And I tell them sex can be the most fantastic thing you do with your body and that it can also be the worst: it can make you feel abused and used. I tell them it's best to be old enough to appreciate it."

Why is Smith - 26 and about to get married - so passionate about all this? "Because my own sex education was so crap," she says. A spell as a sex educator in South Africa also showed her how high the price of sexual ignorance can be.

According to Katrina Mather, 16 and Robert Sassoon, 18, of the UK Youth Parliament, all too often school sex education is still poor. They know that from the findings of the parliament's surveys of young people and from their own experience. The parliament has waged its war for compulsory SRE in school through its "Are you getting it?" campaign - and it may get its way. The Department for Children, Families and Schools is currently considering how SRE in schools can be improved and a member of the UK Youth Parliament is co-chairing its review.

Sassoon says the young are particularly disappointed by the lack of relationship content in SRE. Simon Blake, chief executive of Brook, agrees that that is one of the main issues. Brook does still deal with teenagers who believe that you can't get pregnant the first time and that an empty cheese-and-onion crisp packet - and apparently it has to be cheese and onion - makes a good condom. However, it is far more common for teenagers to become pregnant or contract an STI because of a lack of understanding of the complex emotional and social factors around sex.

There's often a denial of feelings that are new to young people - and about which they have no one to talk to. "So they don't plan for sex because that would mean admitting they were thinking about it and they worry that they would be judged on that," says Blake. "They don't have sex because they think they would enjoy it and are ready for it but because their mates are having sex or because they were drunk."

Apter points to the importance of emotional understanding in the protection of girls. "You cannot protect them by limiting their knowledge," she says. "What has been found to help them is talking to them about desire and the role that plays if they are going to have sex. There are studies that show that girls who have a deeper understanding of desire are less likely to engage in casual acts of sex. They understand that sex is a choice, not something you do because someone has asked you to."

If SRE does become mandatory, parents would still have the right to withdraw children from lessons, but children and young people would have to be taught to minimum standards. Sassoon says it's illogical for opponents of SRE to argue that schools are hijacking a parent's right and responsibility to instruct children about sex. "The fact is that some parents are just not comfortable talking to their children about sex and many young people aren't comfortable with it either," he says. "My mum has been trying to have the sex education talk with me for years and every time she opens with 'Have you got a girlfriend then?', I leave the room. Of all the people I might talk to about sex, my mum is bottom of the list, though I understand it's the opposite for some people." And his dad? "He's lackadaisical about it," says Sassoon. "'Don't get anyone pregnant,' is what he would say."

Sassoon thinks it is the right of children and young people to get SRE at school, but from his own experience, he says some teachers are "too embarrassed" to teach it well. He argues that it should only be delivered by those trained in PSHE or SRE. Mather agrees that parents are just not comfortable doing what is necessary. "I think it's the effect of years and years of failed sex education," she says.

Eve Cooper, 14, a Highlands pupil, agrees that when it comes to sex, sometimes teenagers need to talk to someone other than their parents. "The great thing about Miss Smith [Alice Smith] is that nothing embarrasses her," she says. "You can ask her anything." Cooper says she understands the conflict that can arise between parents and teenage children over the availability of advice on sex at school. "I can understand that parents would want to be the number-one advice givers," she says. "But I also understand why a child would want to seek advice in certain situations from a professional who wouldn't get angry and have a go at them. I also don't think there should be pressure on older teenagers to tell parents things if they can help themselves in a situation."

Cooper's father, Phil, works at Highlands as the gifted and talented coordinator and he's very happy with the sex education his daughter gets at school. Does he share the worry of other parents that their children will be referred to sexual health services by school without their knowledge?

"I've thought about how I would feel if my children sought advice without coming to me and I've concluded that their health would have to come first," he says. "But that's my view from a calm, rational perspective, and a hypothetical position. It's a tough one for parents".

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