Ya know, all of the advice you've been given about "go to the media, etc." has been offered by people who have nothing to loose if this turns out bad for you or your school. They won't have their prom canceled, they won't be turned away at the door, they will face no retribution for confronting your principal, they will not be branded as a trouble maker. In other words, if you follow their rabble rousing advice and things go south, you are the one who will take it in the neck, not them.

From what you've posted it looks as if you went through channels and had your request turned down. The basis for that refusal is as unknown to me as it is to everyone else who has offered their advice. For all any of us know yours may not have been the only request to wear "festive ethnic garb" and the school's decision is based solely on a desire to prevent the prom from being turned into a costume party. Not that it matters what was the basis of the decision. The die has been cast, your number didn't come up, and it's time to accept that the mature course of action is to acknowledge the fact that you won't go kilted to the prom, and get on with your life.