Just tell the admins exactly what happened. I was landed in a similar situation freshman year, except I directed my teacher to my school's wikipedia site, were someone had uploaded rather graphic photos. My teacher was using the computer projector onto the main board, so the entire class got a rather disturbing show.
Just explain that it was an accident, maybe write an apology letter to your teacher.
But I must ask the question: Doesn't your school have a content blocker? If it does, how did the site get through?? Weird.