Topic of the day: the forbidden prom

This is one of those story where I don't feel anyone is on the right side: a private (Christian) school student was told by his principal that going to his girlfriend's prom (kinda like graduation party outside US) was against the rule of the school, which forbids dancing, rock music, hand-holding and kissing (some of all of which were likely to happen there!) And he still went and had to face the consequences: banned from his graduation ceremony. On one hand, rules like that are just way too conservative (it's not the Middle Ages!) I always think a student's behavior outside of school should be none of its business. On the other hand, the rules were known before the student's parent enrolled him in and he agreed to abide by that. I don't see how he could complain now.