I agree with your first point but not really the second. I don't see the big deal about the pledge; afterall, it's not a legally binding agreement. I also don't think all the students would decided against having sex because of the pledge they made at 12. I don't see it any different than any other pledge they made be asked to sign (the drinking and driving contract for example).
I not saying that they should drink and drive, but rather that this type of agreement doesn't really carry a lot of weight for many people.
Which is exactly why these pledges have no place in schools and why my children do not sign them.