Really, Lass, how did you expect me to reply? To say "Ah, you got me, there was no good reason for girls to be allowed to wear trousers, we were just kicking up a fuss for the hell of it to be awkward"?
Time and time again it comes back to the same sad trope that women should not challenge the status quo unless it is for a very very good reason, and that just having a bit more choice, or wanting to do away with an outdated practice or idea is not good enough reason.
I find it ironic, given that women are supposedly the emotional, less rational half of the species, that in order to make any justification for change we are challenged to the nth degree to prove why it is logically necessary. On the other hand, most patriarchal traditions are justified by no more than "This is the way we do things, so why change?"
Rather than asking for reasons why girls should be allowed to wear smart uniform trousers, why not put forward the logic for why it is necessary for them to wear skirts?