I repeat trousers are UNISEX, not fucking gender-neutral
Yes trousers are unisex
In fact many girls wear trousers in the firm of Jean's and leggings all the time.
However school trousers specifically do not seem to be designed with girls in mind. At all. The only ones that even have a hop in hell of fitting properly are skinny style trousers. However those are often the one type of trousers not allowed
I have 2 dds and I can count on one hand the number of pairs of trousers that have fitted. Comfortably. Without cutting them in half lengthwise, dragging across the floor , having yards of material in the wrong places. I mean it takes talent to design trousers that can simultaneously have enough spare material hanging off them yet you cant bend your knees 
Given 2 of the 4 pairs we have found are skinny style and the next size up of the one that wasnt is far too big for dd then either I have the most abnormally shaped, children known to man or these trousers have never near a pre teen girl at any point in their design and creation .
My dd would not be in skirts if these trousers actually fitted and I've no doubt that would also be true for many many many girls.
And now they take the back.up.away..