“It’s 1pm and already we’re talking about masturbation,” declares pint-sized bodybuilder Antonietta Moch, with a broad smile across her face.

“Masturbation is very private,” she hollers across the classroom. “Masturbation is about having sex with yourself. It’s about pleasuring yourself.”

“Don’t put it on Facebook,” she says, to uncomfortable giggles from her audience. “And when it comes to having sex, that’s private too.”

Moch is giving a sex and relationships tutorial to teenagers at a college in Milton Keynes. She is honest and informative and everyone in the room leaves having learned something new.

Under current arrangements, not all children get sex education at school. Free schools and academies, which are not obliged to follow the national curriculum, can opt out and the pressure on the curriculum at other schools means provision is patchy and in some places non-existent. But momentum is growing for change.

On Friday MP Caroline Lucas’s private member’s bill to ensure that sex education is a statutory requirement in schools will get its second reading in the House of Commons. The personal, social, health and economic education (statutory requirement) bill will make PSHE, including sex and relationships education(SRE), a compulsory part of all state education. It also requires PSHE to include teaching on ending violence against women and girls.

This week MPs on the education select committee, who are considering the effectiveness of current PHSE and SRE, were told that the quality of sex education in schools was a postcode lottery. Some youngsters were growing up not being taught even the most basic information about their bodies, while others had no understanding about rape and sexual consent. In the absence of a comprehensive sex education, young people were relying on myths from the playground and increasingly on pornography and the internet to fill the gaps.

Moch, who works for Brook, which provides sexual health services and education for young people across the UK, leaves nothing to chance with her class. To break the ice, the eight boys and five girls – aged 17 and 18 – are asked to come up with a variety of names for the penis and vagina. They’re much better at penis names than vagina names. “But the vagina is just as important and powerful and amazing as the penis,” says Mochcheerfully. As she speaks she is surrounded by three erect plastic penises –blue and brown – which stand upright on the desk in front of her like small Easter Island statues.

The session ranges from why young people have sex – “because it feels good,” offers one boy; “to get it over and done with,” counters a girl – to the impact of alcohol and , peer and partner pressure. They talk about condoms and lube, porn and anal sex and spend quite a bit of time on rape and sexual consent. There is a video featuring a young couple about to embark on a relationship, comically sorting out consent with the help of their lawyers.

Sexual consent is a key concern, with reports that sexual harassment on university campuses in the US and the UK is rife. An NUS survey last month found that 37% of female students had been groped or inappropriately touched.

In a packed, laughter-filled, hour-long session at the Bletchley campus of Milton Keynes College, the class also manages to learn that five and a half inches is the average length of a penis; men ejaculate at 28mph (the world record is 43mph); and the average age for having sex for the first time is 17.

Three out of every 1,000 people in Milton Keynes is HIV positive, they are told (“so it’s very important you are using protection”) and ; you can’t lose a condom in a vagina and chicken juices can damage your condom, so wash your hands after eating at Nando’s. Once everyone has had a thorough briefing on how to put on a condom, the lesson concludes with a condom race using the Easter Island penises, with pants for prizes.

Moch is passionate about her work and is concerned that not enough schools are providing a good enough sex education. “Some schools are really, really good and they get us in and let us develop what we feel is best for the age group,” she says. “Some schools have very limited time and they get us in for half an hour if we are lucky, just to talk about the basics – who we are and where you can go for condoms. Some don’t get us in at all.”

Before the tutorial she holds a drop-in session in the student common room where she sits surrounded by condoms and leaflets on oral sex while girls question her. “Why can you get married at 16 but you can’t watch porn until you’re 18,” asks one. Another asks about the anatomy of the clitoris; there are discussions about smear tests (“That’s disgusting, I would never do that,” says one girl) and chlamydia tests, and more talk of the impact of porn, which seems to be a big deal. Boys watch it on their phones in the common room, says Moch.

“Boys try and become like the porn stars,” says one young woman. “They say they are going to ‘beat’ or ‘bang’ a girl. When you begin to do it, they get quite aggressive and put their hands round your throat.”

So do these young women feel well equipped to deal with this? Some say they didn’t get a great deal of sex education at school and welcome the opportunity to talk informally with someone who is not their teacher. “That’s just embarrassing,” says one 16-year-old. “You have to see them every day afterwards!”

• This article was amended on 24 October 2014. An earlier version said that the Bletchley campus was near, rather than in, Milton Keynes.