I'd prefer teachers, along with their parents and the rest of the nation, teach them about abstinance. THAT'S the right way!
Emphasize it however you'd like, they're your kids. Keep it honest though, and be ready to answer any questions that they might have. Lies and exaggerations (even when they're told for good reasons) have a way of boomeranging on you. I still remember the debacle of the DARE program, and even 20 years later I have a hard time thinking of any other class where I've been told such a pack of lies.