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!
"That you are here - that life exists, and identity; that the powerful play goes on and you may contribute a verse. . . What will your verse be?" Robin Williams as John Keating in "Dead Poets Society"