A national study of the sexual attitudes and lifestyles of almost 19,000 British adults showed that men who felt they had learned most from school lessons were less likely to have had sex under the age of 16 than those who had most of their information from other sources.
Women who felt they had benefited from sex education were no more likely to have lost their virginity under the age of consent.
After sex education at school, both sexes were also more likely to have used some form of contraception, probably a condom, when they first had intercourse, according to work by Dr Kaye Wellings of the London School of Hygiene and Tropical Medicine.
The other study, of a specially-developed sex education programme in Devon secondary schools, found that by using a mixture of doctors and older students to provide targeted lessons and discussions, sexual activity under 16 was less likely among those who had been through the programme.
Its graduates were also likelier to disagree with statements suggesting that sex was beneficial for teenagers and their relationships, and more likely to answer correctly questions about contraception and sexually transmitted diseases.
The two papers, which appear in the British Medical Journal, should confound right-wing critics of sex education who claim that it encourages promiscuity and early experimentation. In a book published only last month by the Family Education Trust, author Robert Whelan claimed 30 years of sex lessons in schools was marked by increased teenage pregnancy, which was "hardly a record of success".
The paper by Dr Wellings acknowledges that the average age of first intercourse dropped by four years between the 1950s and the 1990s. Early intercourse, she says, is more likely to be accompanied by feelings of regret, larger numbers of sexual partners and less protection from unplanned pregnancy.
However, adults who claim the bulk of their sexual knowledge came from school lessons - particularly men - were less likely to lose their virginity before 16 and more likely to use contraception.
She concludes: "These data provide no evidence to support the concern that provision of school sex education might hasten the onset of sexual experience. These findings have important implications for the provision of sexual health education and highlight the need to carry out prospective and randomised studies of the impact of sex education."
Dr John Tripp, senior lecturer in child health at the University of Exeter, who heads the Devon programme, believes that effective sex education, leading to the postponement of first intercourse, would therefore have medical and social benefits. Teenagers losing their virginity before 16 not only had more partners during their lives, but more partners each year. They also started having sex earlier in new relationships.
"Our educational intervention is expensive because it involves external staff - but the benefits last for the rest of your life," he said. "Kaye Wellings shows in group terms, if you first have sex under 16 you have twice as many partners by the age of 50 as those who didn't."
The Devon programme, called A Pause, has teachers taking specially-constructed sex education lessons, concentrating on biological aspects, until pupils are 13. In Year 9, the external A Pause team - a doctor or nurse and a teacher - will take over.
Anne Weyman, chair of the Sex Education Forum, said: "These findings give a boost to teachers and health professionals who have been providing good quality sex education to young people. They also refute the recent extraordinary claims made by anti-sex education groups that sex education encourages promiscuity. "