Options:
-have your phone recording sound all day every day, to record the weird conversations and inappropriate stuff for evidence
-Film him creeping
-Skip his class from now on. Then if you get in trouble, tell them you don't feel safe
-isn't there a counsellor with who you can discuss this confidentially? maybe he has done it before
-tell your friends to really pay attention
-are you sure you are the only one?
-look around when it happens, you can't be the only one that notices. If someone else does go talk to that person for advice.