Yea, I was one of those kids. I literally thought I'd go to Hell if I had premarital sex. Hormones got the better of me, though!
So what do you mean "properly supported"? By parents, religion, school??
Well, proper support doesn't mean teaching you you'd go to hell if you had sex. For now, let's just leave aside the fact that it isn't true on an accurate reading of scripture.
By "proper support" I mean something extremely comprehensive. It includes such things as accurate and forthright teaching about the ins and outs of sex. Of course, I don't mean just the physical part, but the emotional and spiritual sides of it as well. It also means providing an environment where the child can safely admit that he or she has sexual feelings (even non-mainstream ones). (This environment need not be home -- it could be something else.) It should also provide means for coping with those sexual feelings beyond praying for forgiveness in your bedroom. It also means giving the child confidence that if they screw up they won't get disowned (part of the reason I don't like hellfire dissuasion techniques). AND it means providing support for girls who get pregnant so they can keep their children or give them up for adoption. It means providing ways for girls who have had abortions to rediscover communion with God and the church. It means giving children outlets so that sexual tension can be alleviated without recourse to sex.
It also may mean segregating boys from girls at school. I'm a big fan of all-girls and all-boys schools. There's no evidence that such environments stunt childrens' social skills later in life. And there's plenty of evidence to show that children raised in such environments have a much lower instance of extramarital sexual activity. Indeed, if I were in a position to do anything about it, I'd make all publicly funded schools separate for boys and girls starting from middle school. Take that!
Failing that, the next best thing you can do is simply provide outlets for that sexual tension -- sports, hobbies, extracurricular activities and so forth.
Yes, there will always be children who stray. My proposals wouldn't reduce teen sex, let alone unwanted pregnancies or disease, to zero. But it would make a serious dent. The problem is that, in the main, most churches provide pretty lame youth groups for teenagers. The youth group teaches that children who have sex go to hell and otherwise ostracizes youth who begin to show interest in sex. It's a stench. And it's part of the reason people view the Christian gospel as irrelevant and ineffective when it comes to sex.