I am sorry but a school has no authority to require girls to dress in ways which make them feel sexually exposed, which is the case with jumpers in situations like the Opening Poster describes.
There is no justification for making girls feel sexually exposed and there’s no good to be achieved that way, no good to be a fair price for that.
Especially a Catholic school should make sure girls aren’t feeling sexually exposed.
Making girls wear what makes them, and makes them feel, sexually exposed, for the sake of appearances of conservatism (and modesty therefore?) in clothing, would be hypocrisy.
Also, if the uniforms show too much and the girls don’t want to wear them, isn’t enforcing such uniforms actually illegal? A form of sexual molestation?
Not like the kind of jeans girls typically wear are much better when it comes to fitting and showing the contours, but at least trousers don’t ride up.