For some reason, I doubt he was stopped from going to the graduation for the tray incident, it was the language he used in that meeting. Two wrongs do not make a right and if he'd kept his mouth shut, they could/would have had a case against that principle for his behavior. The kind of language he used is not acceptable anywhere, anytime and it does have consequences and he knew it. If it is all as you reported, I think the principle did the right thing.... But I would also add, as his mother, if a principle started yelling at my chikd, I would have took my child and walked out of that meeting.

The whole 21 years I worked at school, the senior prank was to toilet paper the english teachers house... it was a school tradition.... wonder if that is still going on?