You are all invited...

[Ugly Bear]

my principal would rather me not wear my kilt...

To a graduation? You're wearing a graduation gown, right? With the graduation gown, it's just about impossible to tell what you're wearing underneath. So there should be no problem.

Really bringing the point home: At the conclusion of the ceremony, he'll no longer be your principal. So his actual authority is going to go up in smoke.

Even so, this is an occasion to dress respectfully and remember that this is a semi-formal event. Utilikilt and wife-beater? No. 8-yard traditional, shirt & tie, ghillies & hose? Yes. Full Prince Charlie? No, too much.

However, as this is still a school event, best to leave the sgian dubh home. If you bring a "weapon" to the graduation, that's asking for trouble from people with more authority than your principal.

... wear a kilt to protests the principals bad taste and stereotyping.

What was the stereotype the principal used?

BTW, congratulations on graduating.

[Big Paul]

Congrats! I wish I could make it.. Just a little to far.. Rats!

Best of luck!

[Iolaus]

However, as this is still a school event, best to leave the sgian dubh home. If you bring a "weapon" to the graduation, that's asking for trouble from people with more authority than your principal.
What was the stereotype the principal used?

Does that law still apply at a private school? I'm not sure; it may only apply to students if it's in the school rules, but not to visitors.