I truly am sorry I cannot be there to honor your fine achievement and support you and your father with yet another kilt. It is a sad state of affairs when an administrator attempts to abuse their position. Surely he/she understands that your intention is to honor the occasion. I can understand if you were intentially going to wear clothes that would mock the event and spoil it for your classmates and their parents but this does not appear to be the case. I can tell you as a former teacher in both public and private high schools and presently a college professor that if I was to hear of this happening where I teach I would be on the front row in my Ferguson tartan under my academic robes (and my degree allows that the academic robes be open).
As I am sure your father has suggested - Be always the gentlemen and respectful yet stand by your principles. Note the spelling.

Cheers! Bill