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.

There’s a special providence in the fall of a sparrow. If it be now, ’tis not to come. If it be not to come, it will be now. If it be not now, yet it will come—the readiness is all. - Hamlet, Act 5, sc 2