I agree that the main thrust should have been "We have rules, please follow them" but
it is a safeguarding issue and one I have recently had experience of.
You see it is not necessarily the girls that are being protected. It could well be all male staff.
We have recently had allegations made against a member of staff. Young lady, short skirt, slouching in her chair, feet out from under the desk, sort of sat sideways in her chair. When he asked her to sit up and put her feet under the table, out of the walkway, health and safety etc, she screamed blue murder at him and left the room. She made allegations that he had been looking up her skirt and he was immediately suspended.
Now, it is possible to say that she should have been able to wear whatever skirt she chose, it shouldn't sexualise her, etc, etc, etc. But, having seen that pose before, many, many times, it is also all too possible to understand how helpless he will be against the allegation - after all, in noticing how she was sat he probably would have seen her knickers!
Suspend your outrage, not everything is about 'victim blaming'. Sometimes there are perspectives you just haven't considered!