I do not know what kind of school this is, but imposing unilateral rules without any reason isnt really such a good idea.
If its a more charedi school, probably wont make a differnce as most will obey, but more modern ones the girls will want reasons for the rule.
You also dont want people obeying rules in front of the principal and then disobeying them behind their back. That does not create such a good atmosphere (I have seen this too many times)