Sex education in schools may be more effective if delivered by pupils rather than teachers, according to Edinburgh University research.
A study of 15-year-olds found they were less likely to have intercourse if school was their main source of sex education, as opposed to parents or friends.
But researchers suggest that school-based sex education could be even more useful if pupils were more at ease, as they are when discussing such issues with friends.
"Incorporating peer-led sex education may be beneficial in teaching young people about sexual matters in school," they state.
Previous research had already "acknowledged the place of carefully- designed peer-education programmes for improving sexual health among young people".
Some 49 per cent of boys and 34 per cent of girls report schools as being their main source of information about sex, but this is a relatively recent phenomenon: an equivalent 2002 study recorded figures of only 29 per cent and 23 per cent, with friends a more common sounding-board.
Boys who report school as their main source of information are far less likely to have had sex (8 per cent) than those who cite friends (41 per cent) or parents (42 per cent).
Girls who report school are far less likely to have had sex (23 per cent) than those who get most information from friends (42 per cent), although those who have parents as their main source are even less likely to have done so (22 per cent).
The study, funded by NHS Scotland, also finds that pupils who receive sex education are less likely to have negative views about condoms.
School has overtaken friends as the main fount of knowledge because there is an "increasing prevalence of information provided about sexual matters to young people at school". A 2005 drive by the former Scottish Executive to increase sex and relationships education is cited as a possible turning point.
A Scottish Government spokeswoman said the report showed schools could make a difference in educating young people about relationships, sexual health and parenthood, although they could not do it alone.
The possible effect of sex education in schools on sexual activity is described by the researchers as "encouraging", but they caution that there may be other factors at work.
It is too far a stretch to deduce that all teenagers having sex, or considering doing so, have not had enough sex education at school. Instead, it may be that sexually-active teenagers list friends as their main source of information because they have a more pressing need for advice, and are happier asking those closest to them rather than teachers.
Some 76 per cent of girls find it easiest to discuss personal and sexual matters with friends, as opposed to 1 per cent who find teachers the easiest. The equivalent divide among boys is almost as wide, at 71 and 3 per cent.
Previous research found that pupils' embarrassment and teachers' discomfort was hampering discussions about sex and relationships in schools.
"Improving teacher-pupil communication about sexual matters may further increase the benefits associated with sex education in schools," said researcher Jo Kirby.
The research, involving more than 2,000 pupils in 300 Scottish schools, was carried out by Edinburgh University's child and adolescent health research unit. It is part of the World Health Organisation's long-running Health Behaviour in School-aged Children study.