You're giving me a bit of a headache.
The feeling is more than mutual trifle.
If we don't know that the boys need to have the uniform code reiterated (as nothing like this is mentioned in the OP's post) what on earth are you complaining about? If there was a double standard on display I would agree with you, but so far all we know is that the school felt it necessary to speak to the girls but not the boys.
If we don't know that the boys don't need the uniform code reiterating, you shouldn't have said that they don't.
And yet again, there is a double standard on display because, based on what we have been told, the school told the girls that they need to consider the males around them and has said nothing of the sort to the boys. There is no way to make that not a double standard: you can try to excuse it, but you cannot say it is something other than what it is.
Plus, there is nothing wrong with telling them that their breaches of the uniform code are having a negative effect on the boys. The code is there to make sure everyone can get on and learn, so if one group is at risk of not being able to do that because of the actions of another group, telling them so is just telling the truth.
Yes, there is everything wrong with it.
This is happening in a wider societal context which tells women that they need to consider male feelings about their behaviour and dress. The same policing does not exist for men. It is harmful to women and girls. These things are all facts.
Perpetrating it in schools is unacceptable, particularly when they could simply have been told that the uniform rules are non-optional. You cannot, however much you might like to, divorce this from the wider societal context.
But I thought bertrand's nothing sexist here folks, move on, summed it up quite well.