In the end it really comes down to how passionately you want to see this thing through and to what end you are willing to go. Although it is wise advice to choose one's battles carefully, I also understand if someone would want to make this their last stand. If this were so, I would probably approach it thusly.

I would start out by meeting with the principal and talking to him again, showing him the photo (or similar ones) that Jock Scot did, and speaking about Celtic heritage. Failing that, I would mention that it would, in fact not be lawful to bar me from wearing a kilt to the prom, citing precedence (Utah). I am reminded of a line in Fiddler on the Roof, when the young couple approaches Tevye to announce that they were getting married and they tell him that they are not there to ask for his permission. They are there to ask for his blessing. I would tell just that to the principal. Meaning, in other words, say "yes" or "no" I'm coming kilted regardless. I would rather do it WITH your okay than without... But if it's without and I am sent home, there's going to be a storm of legal and media backlash over this.

Again, that is if I wanted to make my stand here and not back down no matter what.

Otherwise, the advice from my previous thread still stands.