I can't imagine having sex in high school, but I realize I'm in the minority, haha.
Sex is a tricky thing. Basically, anyone you ask about sex will probably have a different opinion on when it's okay, when it's not, what it means, what sex actually IS, what losing virginity is, etc.
I think if you go into sex in high school expecting a super-intimate emotional relationship as a prerequisite of sex, you'll probably not get that, and be disappointed. You can achieve that, of course, but like fidelia mentioned, lots of people at that age aren't quite that "mature," for lack of a better word. Sometimes people in high school think when the partner wants to have sex, it's to have sex, whereas the other person is thinking, "Oh! We're serious now! He must really really love me." Which isn't always the case. At the same time, I can name several people who have had intimate sexual high school relationships that have carried over to college and to adulthood and remained successful (from what I can see). They're just not the majority, though.
I also know people who also just don't have a huge emotional-attachment to sex itself. They can have sex for fun, or have sex in a good relationship, but they don't let the sex define how they feel about someone. Sure, the sex is better if they're emotionally tied to the person, but the fact that sex was in the relationship isn't why they're emotionally attached. It's a separate thing. I think this mindset is what makes teenage sex work. (I'm not one of these people, so abstaining from those kinds of relationships in high school was a good call on my part!)
Basically: If you expect sex to be a result of emotional intimacy and love, teenage sex is bad since getting that is unlikely. If you expect sex to just be sex, it's neutral in my book.