There could be many reasons why the student didn’t have underwear on. The principal could have simply called her parents to come and pick her up and explained why. There is no reason for her to show off her private parts to people in a room. I would definitely say what the principle did was inappropriate and wrong but I don’t exactly define it as abuse.
whatever. I am betting the student wanted to show off her privates and the principal was irritated that the student had done it again