Girls roll up their skirts for two reasons. 1, because some of them actively want to look sexy to boys and men and to 'prove' how grown up they are. 2, because the small number of girls who don't roll their skirts get bullied or teased about it. So most follow the crowd just to blend in.
It is not unreasonable to have a dress code in schools which protects children and teachers. Nobody is exaggerating, there are large numbers of girls walking around with skirts so short you can literally see their buttocks. And lots of them look uncomfortable (holding onto their bit of skirt with one hand at all times) which tells me they'd rather not but are bowing to peer pressure.
Staff do not need to be deliberately looking up girls skirts to see their underwear. It's right there, especially when going upstairs or sitting on the floor. This is not acceptable in a workplace, and is contrary to the school's duty of safeguarding.
I think having trousers for all is a more sensible choice, ideally they could offer different cuts to suit different body shapes.