Time to call the Special Victims Unit; you definitely seem to have been put in a mandated reporting situation. Do it for the kids. He may already have accumulated a history of complaints or even offenses. If this guy is far gone enough to behave as he has in his position of responsibility, then what else might he be capable of? High school principals should be squeaky-clean. There's no burden of proof on you; just find a friendly detective, ask to speak to them in private, and tell them what happened. Whatever you deleted, you deleted; if they can recover it, they will.