Jennifer Roback Morse explains it all:
A 2002 study published in the British Medical Journal examined 26 programmes that included school based programmes, multi-faceted programmes, family planning and clinic based programmes, as well as abstinence programmes in the US and Canada. The results: “The interventions did not delay initiation of sexual intercourse in young women or young men, did not improve the use of birth control at every intercourse, or at last intercourse for either men or women, did not reduce pregnancy rates in young women ...
Another study of a very well-designed and well-delivered sex-ed programme in Scotland was also published in the BMJ. The result: “When the intervention group was compared with the conventional sex education group, there were no differences in sexual activity or sexual risk taking by the age of 16 years.”
Then there is Douglas Kirby’s 2001 survey of over 300 programmes of all sorts. “Most studies of school-based and school-linked health centers revealed no effect on student sexual behavior or contraceptive use.”
So what does work? Programs that "include substantial after school and community-based components," which often involve activities that are only indirectly related to sex - "enrollment in fitness and dance classes.... tutoring, working as a teachers’ aide or working in nursing homes ...group discussions, journal writing or papers." Programs that increase "connectedness to caring adults," in particular. None of which should be surprising: Given that having married parents seems to be the strongest factor in kids delaying sex and avoiding pregnancy, it stands to reason that only deep, comprehensive, in loco parentis interventions would have a significant effect on teen behavior.
This would seem to suggest, in turn, that the best way to approach sex ed is to invest heavily in precisely those sort of programs - with condoms or without them, depending on what the locals want - in areas with widespread family breakdown, and otherwise to leave well enough alone. The sex-ed classroom is one culture-war battleground where the data suggests that the battle isn't worth fighting, for either side, and the sooner we can call a truce the better.