If the school has any written policy about culturally significant or symbolic clothing being an exception to the dress code you can always use that to boost your argument. Despite the impression given by most public institutions, the kilt is no less a culturally significant item than a turban or a yamika. The only downside to using this argument is that it applies more to traditional kilt and wearing them in a traditional style, so it probably wouldn't open the doors to wearing something like a Utilikilt.

Hopefully your teachers and administrators haven't been brainwashed into the all-too-common misconception that the word "ethnic" either can not apply to things originating in Europe or that it means "non-European in origin." Then you're saddled with correcting their misconceptions before you can move on to getting permission or changing policy. Been there. Its a struggle. Good luck.