...to be kilted at prom...

[RockyR]

INITIAL POST EDITED: Rethought the comment and changed my mind.

NEW comment:

I'd approach the principle with well thought out reasons including the love of Celtic Culture and your heritage.

Also, his mind may be racing with thoughts of 'kilt liftings' and general debauchery. If you explain that this isn't a 'costume' to you and that you will wear it properly and with respect, he may be a little more apt to allow it. Present yourself as MATURE when discussing it.