I don't (personally) have a problem with him partying a little after prom. I doubt there are many here who could say they didn't. I guess the thing is that there had to be something drawing attention to the party for anyone to get busted.
I had a room after prom, but just envited about 6 people (then threw them out when it was time to do the nasty
).
That said, Tress should hook him up with some councelling. Just to make sure this doesn't become any more of a problem than it already has. There are a lot of things that kids don't know that you learn there that will open some eyes.