I don't think he did anything wrong I mean you weren't a student anymore if it was your prom, you're both adults.
But I'd hazard a guess that if it got round your school to other teachers etc it wouldn't exactly help his career along, mud sticks and all that jazz.
Tip- don’t drunkenly tell any so called friends naively thinking it’ll be kept quiet cus it won’t, and things get exaggerated and told differently and you can deny it all you want but it wont make a difference.
OP you can PM me if you want to chat and you are genuine XX
EDIT: But if this is someone's idea of a joke I'm not amused…