Teens are dumb.
Teens think they know everything, but they're as bright as a sack of cannonballs. Sure, they may be street smart, or book smart, or (if they're lucky) a little bit of both. They may possess common sense. Hell, perhaps a few will rank as geniuses, acing every IQ test tossed at them.
But when it comes to life, they know zero until they've lived it. Teens just haven't lived long enough to know that decisions they make today aren't necessarily the decisions they'd make a year from now in the same situation. The kids who are shortsighted enough to sign purity pledges or take nude photos/videos of themselves share one thing in common: Both think life remains static and don't fully understand the ramifications of the decisions they're making.
Sure. They may feel like they're the coolest kids in school right now, with their purity rings and what not. But, in a decade or so they'll look back on the little nugget in time that we refer to as adolescence and say, "Wow. Look at that hair. Those shoes. And that purity ring.'
'I was a retard back then."
Here's the crux of the matter: The only way teens learn anything is by screwing up. So, educate them. Warn them of the consequences of their decisions. And let them make some stupid choices. If they have any intelligence/common sense, they'll learn from their mistakes.
If not, look on the bright side... the world won't be lacking in amateur teen porn anytime soon.