though i don't know the whole story - my bet is that NO - you were not to blame. she was the adult - and a parent at that - and responsible to keep you safe. you were a kid - legally a child - and not expected to be the one setting the tone for the relationship. 15-year-olds may be physically "mature" but are definitely NOT emotionally or mentally mature. even if you think you could have stopped it, the power balance was not on your side. authority figures weild much more power than kids.
IT WAS NOT YOUR FAULT!
"the scariest thing about abuse of any shape or form, is, in my opinion, not the abuse itself, but that if it continues it can begin to feel commonplace and eventually acceptable."
- Alan Cumming, "Not My Father's Son"