How he looked isn't the point. Had the principal said "he's not dressed formal enough, or neat enough", that would be understandable. Require him to be more formal (though I doubt he would have been the "worst dressed" anyway). No, it's the fact that the principal considers someone dressed in a kilt to look like a CLOWN! That's my hertitage, and I'll be damned if I'll let someone walk all over it. Had this been some traditional South American, or African garb, there would be large scale public outcry already....
Hoot, mon! If I got on a public bus with a golly-wonking boom box playing the 'pipes, do you think I'd be asked to turn them down? Probably. Cultural insensitivity, indeed!
He did look like a clown! It was disrespectful to YOUR heritage (and to mine!).