hasn't such a study already been done? It obviously wasn't a prospective because as you said, it's unethical, but I'm pretty sure people with abstinence-only sex ed have a higher pregnancy rate than people with more comprehensive sex ed.
This is kind of old
http://ari.ucsf.edu/science/reports/abstinence.pdf
But here is a very recent one
Skyscape: MedAlert: Mobile medical references for Palm, Pocket PC, PDA, Tablet PC, Windows Mobile and Smartphone.
this is interesting too:
Oral sex lessons to cut rates of teenage pregnancy | UK news | The Observer