The bottom line is that you're there, and we're not. It's easy to say "screw the grade point average," when you don't have to live with the consequences. Personally, I think you'll be fine with wearing the jacket and tie with the tartan kilt, based on what you've reported of your conversation. However, again, I don't have to live with the consequences if I'm wrong.

I think the plan of action you've outlined, dressing in the kilt, checking with the teacher, and being prepared to change into trousers if he doesn't like it, is the best plan of action.