Last year in southern Alberta Hamish Jacobs had the same issue, below are the links to the facebook page created a few news links and an online petition. I hope this helps, I'd say the best thing would be to talk to the school board and see if you can solve the problem before the big day, explain to them the importance of your family heritage and bring photos to drive the point home, also it may help to assure them that your son will cover up underneath his kilt.

Facebook:
http://www.facebook.com/#!/pages/Ham...14277515281104

The Calgary Sun:
http://www.calgarysun.com/news/alber.../14049921.html

The CBC:
http://www.cbc.ca/albertaatnoon/2010...-his-kilt.html

also an online petition http://www.gopetition.com/petitions/...r-no-kilt.html