Unfortunately we cannot "stop" them. All we can do is prevent and educate as a society, but as parents (if you are one) tell them your expectations and do not sugar coat, but do not use scare-tactics either, about the consequences.
Children are having sex younger and younger because sex is taboo. Noone wants to talk about it, but everyone does it. We are constantly bombarded by images, innuendos, and words.
If you look to Europe, teenagers do have sex but the rates of very young teenage sex are nowhere near the United State's. And it's probably because there is free access of information about sex and contraceptives. Parents are more open-minded about discussing it with their children. People are aware of what it is, and it's almost analogous to the binge-drinking situation what we have in the US. In Europe, college student binge drinking is nowhere as bad as American college student binge drinking, and it's because parents have made that repoir with their children about alcohol, instead of treating it taboo.
It's psychological really-- the more forbidden something is, the more we want it. The best we can do is if we are parents, approach our children about the topic in a way that is meant to educate them about the facts and inform them about healthy and responsible expectations. NOT waiting for the schools to do it for us!