Looks like a sincere and reasonable letter. If your principal had any sense, he would make this a "non-issue" by just saying "okay"...one thing to consider is that another faculty member denied your request...this puts that faculty member in a bad spot insofar as he looses face if he's overruled by the principal. I'd take this into consideration and try to involve that faculty member in the process and maybe try to sit down with both him and the principal and make it clear that you appreciate his position but that you respectfully disagree with it and you want to work this out to everybody's satisfaction. Most important is to communicate your sincerity here and not let it look like you're just trying to win this one just for the sake of winning; that your desire to honor your heritage is what's behind the request to wear the kilt.

Best

AA