One of the biggest problems with the USA education system is they rely on scare tactics for things like sex and drugs. They try to coerce students to their point of view on it by showing them only the consequences of what happens if they don't. However, some student or another will try it out anyway, and odds are, suffer no consequences for that trial. The teachers never explained that it was a RISK, a chance, not a guarantee of consequences. So, the student figures they were either lying or exaggerating or may just adopt the old "it won't happen to me" syndrome, and all the sex/drug education class taught him is essentially wasted.
The other problem, of course, is that these things should generally be taught by parents. No one teaches them how to teach it though, or gives them any reason to do so; so of course the task is shunted off to the schools for most.