Hmm, I'm going to add my two bits. I agree with Dallas that honest conversation and ongoing support about sexual issues are important for teenagers whether they are sexually active or not, and I definitely agree that some high-schoolers have sex because they think they "should", not because they really want to. However, I think a big part of the problem with teen sex comes from areas that haven't been brought up yet on this thread. (Apologies if they did and I missed them!)
First off, high-schoolers are often at a point in their lives where they want to be more independent from their families. They want their own lives, schedules and relationships, and even if they have wonderful relationships with their parents, they don't necessarily want to talk about sex with them. I would recommend for any high school to have a counselor who's reasonably young and connected to youth culture and who knows how to deal with sexual topics, and for all the classes to meet her/him at least once and know that s/he's there to talk to. That way they'll have access to someone who knows their stuff but doesn't feel so much like an authority figure.
When I decided to start having sex (at age 18), I scheduled the doctor appointments, got a full physical and got on the Pill, all without my parents finding out. I felt totally comfortable asking my doctor questions about safety, but there was no way I was going to talk about that stuff with my mom, even though I loved her and was close to her in other ways. Later, when I'd been out of the house a while and we had a more adult-to-adult relationship, I was more comfortable talking about it with her. From what I've gathered from friends, this is pretty typical.
The next thing is that if you're going for abstinence-only, you really need a more convincing argument than "it's dangerous" or "you'll value yourself more". Most teens who hear these reasons will be surrounded by people who are having sex, talking about how fun it is, not getting pregnant, not getting diseases, and seeming to feel perfectly fine about themselves. The ones who do get diseases and have low self-esteem are not as likely to mention those things, so you get a sampler bias. (For the record, I'm fine with premarital sex but I think the high-school social life is pretty warped, so I'm ambivalent about teen sex.) Whether or not schools teach it, the students are going to figure out that the adults who have sex protect themselves with condoms and other forms of birth control, and they're going to figure out that the majority of unmarried adults have sex and most of them don't get pregnant, so the abstinence-only arguments aren't going to look that appealing.
I agree with Dunemeister's position to the extent that if they have a really good reason, high schoolers won't have sex. If every kid under the age of 17 got instant herpes, flu and acne attack whenever they had sex, they'd stop having sex, because it's not worth it. But as it stands now, not all of them are convinced it's not worth the calculated risk. (Edit: a few would still do it and get the herpes, flu and acne, but not very many.)
Finally, it is possible to have healthy, mature sexual relationships outside of marriage, but they need to start with a relationship that's healthy and mature as a whole. Some teenagers (high school age I mean) are capable of that sort of mature partnership, but I don't think most of them are, which is why I'm ambivalent about teen sex. Not really sure how to deal with this in school, though.
One more short thing--abstinence only programs should really stop focusing on loss of virginity. It sends the message that anyone who's had sex at least once, whether they have unprotected sex with strangers or safe sex with a trusted partner, is in the same boat so you may as well not bother protecting yourself.
We may not agree on what the actual consequences of electing each individual are but we totally agree on everything else. 
