I've seen girls walking home from school. They are rolling that high. The school brought in thick black tights because they realised that they utimately couldn't stop the rolling, but they could stop crotches and buttocks from being quite so visible.
Its definitely not just about pervy boys and male teachers. Or promoting modesty. Its about not seeing arse cheeks and worse which is a reality.
These are under 16s. Schools should consider a degree of actual safeguarding here.
After 16 or outside school not in uniform (thus outside the schools responsibility and association with the school) its the girl's choice / the responsibility of parents.