Teenagers want to learn about sex from one another rather than from their teachers, according to the biggest study of peer-led sex education.

The survey of 8,000 teenagers, who had formal lessons about sex from either other pupils or their teachers, revealed that pupil-led classes helped pupils develop better attitudes to sex.

However, they were not more likely to use a condom the first time they had sex.

Significantly, boys and girls who received the peer-led sessions knew more about how to protect themselves against sexually transmitted infections than those who learnt from teachers.

By age 16 only 35% of girls in the peer-led group reported having had sex, compared with 41% of the teacher-led group.

All the girls in the study will now be traced in four years time to see whether their lessons had prevented them becoming pregnant by the age of 20, after a slightly lower teenage pregnancy rate was reported.

The research backs up last year's sex education report from MPs, which concluded that sex education is too often taught by embarrassed teachers.

It was led by Dr Judith Stephenson, from the Centre for Sexual Health and HIV Research at University College London, and funded by the Medical Research Council.

She said: "Previous studies have shown that sex education has little effect on behaviour and pregnancy so it's rather encouraging that our study has shown some effects."

The peer-led lessons were more likely to engage pupils, she said, because young people were more likely to have active lessons, with role play introduced into sessions about negotiating sexual relationships and quizzes to help people understand sexual health issues. "They could also teach in smaller groups, which cut down on the embarrassment factor," she added.

Michael Reiss, a leading specialist in sex education at the Institute of Education, said there was good reason for the teachers' embarrassment. "Most teachers are not specialists in teaching people about sex. It's one of the few subjects where this is the case. You're usually taught history by someone with a history degree and training. If you get taught sex education you're lucky if your teacher has had 20 hours."

The government dictates that all schools have to provide some sort of sex education, though it does not stipulate what that must involve. Parents have a right to opt their children out of sex education classes.