The school should either have the right to set their own criteria in any way they wish, or not have that right.

That's a much larger issue than one based solely on religion.

I don't really care if they kicked the kid out because because her parents are lesbians, or if they kicked her out because she was Polynesian and the school had a "No Polynesians Allowed" policy.

Does the school have the right to set their own standards, religious or otherwise?