If at first you don't succeed, try, try, again.

If you say something enough times, people will believe it no matter how absurd it is.

Keep the talk going with the principal. Buy him lunch one day. ;-) I used to do that when I was in school. But we only had 80 students so it was quite intimate. We didn't have a layer of vice people to deal with!

Seriously, keep the talk going. Bring the topic up every chance you get. After a while people will get numb to the idea and it just won't be an issue.

Personally, I would have just did it without asking permission. If I was really worried about it, I would bring a pair of pants to change into if they banned me from entrance with out them.