“If its a more charedi school, probably wont make a differnce as most will obey”
I think she has said that she lives in Lakewood. A modern school wouldn’t make such a rule and it wouldn’t be appropriate.
“You also dont want people obeying rules in front of the principal and then disobeying them behind their back.”
The Principal specifically said that it’s a school rule and not about tznius so they don’t have to follow it when they are not in school.
That was actually very intelligent, imho, since it avoids the problem you mentioned.
And it also means that the girls don’t need to feel restricted. Of course, they will complain anyhow, but it’s really not a big deal, since they can change as soon as they get home, just like they can change out of their uniforms.