From a personal interpretation:

"Must wear collared shirts, slacks and tie apparel."

I mentally highlighted the term "apparel", meaning the equivalent thereof. Seeing as how the kilt (when properly worn for such functions) is a very classy, sharp and unique outfit, I couldn't imagine that they would turn him away.

But, being the bureaucracy that most school administrations have become, I would follow the advice so sagely imparted by others here. Be prepared, talk to the administration, follow a logical defense, so on and so forth.

That is just my two cents.