My school recently brought in a public speaker who spoke on the dangers of having sex outside marriage. To illustrate her claim, she described how she contracted HIV the first time she had sex. She didn't find out about this until after her current husband proposed to her. She decided to get tested for STDs just to be sure she was free and clear. As it turns out, she was not. It's an unfortunate story though not an unfamiliar one. The gist of her talk was to not have sex until you're married, because sex should only be shared in a committed (married) relationship.
It's not that I think abstinence education should be disregarded. Rather, abstinence only education is ill advised. I live in a realistic world where my students, some as young as 14, are having sex. You can talk about the dangers of teen pregnancy and STDS till you're blue in the face, but that's not going to stop teenagers from having sex.
Abstinence only education relies on fear and guilt. The message is don't have sex because you might get an STD or get pregnant and that would be shameful. Sex education that acknowledges other options such as condoms and birth control drops the guilt aspect but relies more heavily on fear by explaining in detail all the wonderful conditions your genitalia can experience. Bonus points for any sex ed that uses full color pictures.
I have yet to really see a pragmatic sex education approach that relies on maturity and respect. Ideally sex education should neither vilify or glorify sex. The inherent problem with teaching sex education is that it is tied so heavily into perceived morality that any discussion about consequences and responsibility gets lost in the implicit morass of tongue clucking, head shaking and finger waggling.
Ultimately, I don't think schools have enough leverage with students to influence a their decision to have sex or not.
Humans become sexual mature in the early teens, but only hit emotional maturity later on in life with some never reaching it at all. Until the day our physical maturity slows down or our emotional maturity speeds up, we're still going to be faced with this difficult task of teaching sex education to minors. Not teaching it seems irresponsible, but only teaching a narrow perspective on the issue seems just as bad.