Yes, good advice to check with the principal. He/she has likely been dealing with issues of cross dressing within groups of students and sadly, the un-informed put kilts in this group. I am a HS teacher in Minnesota and I mainly only wear kilts to school in the summer to not cause distractions.

I think if you are wearing the kilt in formal settings that is one thing, but a casual kilt might need to have approval first, unless you wish to have a principal, a coach and likely your track team against you.

It is better to ask for permission than forgiveness in this case.

My two pennies.