Mean students! Hormonal kids who are straight up not having a good time! Students who won't interact with a certain subset of student (i.e. they think the UC kids are snobby, they got in a fight with a Ravenclaw once and now avoid everyone from that house when they can help it, they think the prefects are out to get them, etc.) I have a hard time finding enemy-type relationships because everyone is so nice, but even nice kids in middle/high school have those they don't rub right with :P