Crop tops are not allowed at our school and we don't really do non uniform day, there is maybe one a year and that's in winter so we don't encounter this issue.
School trousers are often just not made with girls' bodies in mind. As girls go through puberty, their bodies change they get hips, waists, curves but the trousers tend to be cut in a way that doesn’t allow for that. They’re often too tight in some places and too loose in others, and the fabric doesn’t stretch much. Super uncomfortable and unflattering. (Cue the shrieks from those who say girls at school shouldn’t have to worry about whether their clothes are flattering. Sure.)
Boys’ trousers, on the other hand, are usually looser and more comfortable, with better pockets and a fit that’s just more practical for moving around all day.
There really are bigger problems in society and eduction in particular than policing girls skirts.
These threads come up from time to time, and I’m sure it must be very cathartic for the controlling stickler types who love to keep everyone, especially young teenagers in line.