I have to agree with Nighthawk for the most part that it would be better to just go kilted and not raise the issue beforehand. However, there is the chance that you will be barred from entry. It's just a risk you'll have to assume whether you ask in advance or not! They could say no either way.

Search around the site as there are several threads about the topic from years past. If you decide to ask in advance prepare a small presentation of what you will wear as well as some info on traditional Highland dress for the administration who probably don't know much about it. If they still turn you down, and you aren't a senior and have another chance to go next year, go in a regular suit or tux and take pics of anyone who shows up in "inappropriate" garb such as jeans, pimp suits, girls who are wearing extremely revealing outfits, etc. and use it to support your case for next year...basically saying that it's unfair that you couldn't wear a tasteful kilt but they let people in dressed like this and that. There was a thread last school year on this site about a girl who made a dress out of trash, and another at the same time from a young man who was forbidden from wearing a kilt to prom. It makes no sense that they can do that, but that's just the way it is sometimes.

Also, if they say no(and I really hope this doesn't have to go this far) and other students show up in traditional garb from other cultures I would raise a real ruckus! Last year in Las Vegas I was told I couldn't enter a restaurant kilted as they had a strict dress code. It wasn't my usual casual boots and t-shirt either...it was after my wedding, so I was dressed up. While I was negotiating with the manager they let a guy in who was wearing a turban. Head-wear was forbidden in their dress code as well. I pointed it out and he said that it's "cultural clothing" and is an exception. I told him that my kilt is a cultural item (and casually mentioned mentioned a discrimination suit and the bad press that would go with it) and he let us in immediately! I'm not reccomending threatening a lawsuit, but if they let other students wear things traditional to other cultures they really should allow the kilt.

Hopefully it all goes well and smoothly, but if it doesn't don't back down. Be respectful, follow proper channels, but above all stick to your guns.