Why did the principal side with Rick?

Because that's the way it is in real life. Educators will (almost) always side with the popular kid in any situation unless they see the kid acting out with their own eyes. (as the principal does at the end of the film, and thus sides with Angus)

I was just talking about this to a friend the other day. It seems as if teachers and principals completely forget what middle/high school is like. It was my experience in school that nearly every adult fell for the big popularity contest - and put kids on pedestals who had no business being there in the first place.

But as I've gotten older I kind of understand their position. Popular, mean kids tend to also manipulate adults. They are on their best behavior when adults are around, and as far as I am concerned, overworked, underpaid educators are probably just happy there is a kid in their class who at least pretends to pay attention, does their work, and doesn't kick up much fuss. They also show interest in school functions, student government, extra-curriculars, stuff like that.

Its a teacher's dream.

Unfortunately, many teachers don't look past that facade and notice those kids are torturing others day in and day out. Its the ostrich-syndrome: if I can't see it, it isn't there!

Laura put it a lot better than I ever could. But yeah, most educators will side/sympathize with the bullies than the victims.