Speaking from the UK, so I guess things maybe different over here and I haven't been at school for close on ½ century, but my public school had a strict uniform code, which included the kilt, but there were strict rules on what was worn when and as it was regrettably a single sex school, there was little room for manoeuvre, though it did not stop us trying. Most of the time you had to wear shorts whatever age or size you were.

My best suggestion would be to study the rule book, you never know there maybe an oversight or contradiction which you can use if need be.

Our school had many rules and the inevitable conflicts, three of which included:
you are not allowed to walk outside in the rain, (you have to run);
you were not allowed to run if you were out in the town;
the third specified how long you were allowed to be in the town.

So if you were in town and it started to rain and you needed to get back, then unless you were able to fly, which was not covered by the rules, you were likely to get a beating for either running in town, walking in the rain or being late back. A true lose lose scenario and it rained quite a lot.

Hope you find a satisfactory solution.