The male teachers do not WANT to see any of this, they aren't lusting after these underage but very physically developed girls. But they have eyes, they can see all this deliberate sexualisation, and no doubt it makes them really uncomfortable.
Hell, i'm a straight woman, and it makes ME uncomfortable. It made me uncomfortable when i was their age and it was my female classmates, because it wasn't just what or how they were wearing their clothes, it would be how they physically acted too. They were using body language to flirt and try to tease/turn on the boys, because they liked how the attention made them feel. School is not the place to dress and act like that, so yes, i think its completely justified to ask girls to cover up, because lets face it, the boys don't dress and act like this, it is female behaviour and it is inappropriate to be doing in school.
You've put this brilliantly - yes, it's not about causing uncontrolled arousal in men (or women, come to that) but about making other people feel uncomfortable, and I certainly do when suddenly confronted with lots of flesh where I'm not expecting to see it!
Safeguarding in schools is rightly a priority but teachers must be very scared of accusations being made against them by (almost always) girls wearing revealing clothing. As a pp mentioned upthread, some kids will throw out an accusation of 'looking at my bum/cleavage' etc just to make trouble. Well put it/them away, then.
I think all students should wear trousers to school and something with sleeves. That's a simple rule which cuts out a lot of the nonsense.