For Funks Sake, get off your high horses everybody....this person has come here for some advice, not moral preaching. They have obviously felt that the abuse was significant enough to record it and if it's coming from a teacher, it is unprofesional.
Personally, i'd start with either the head teacher or, if the 6th form college has one, the chair of the board of govenors.
It might be worth playing the recording back to a few older people first though....maybe your parents or some older friends just to make sure you haven't blown things out of proportion.
Good luck