OMFG.
Objecting to this is not the same as saying girls should wear anything they like. It's reasonable to ask both girls and boys to dress decently/appropriately, whether that means not revealing your underwear, having a minimum skirt length or whatever.
It's how girls dress being "unfair on male teachers" that is the problem here.
A) This falsely singles out girls to lay the unfair blame on. What about female teachers (or gay male teachers) teaching teenage boys who may appear attractive or sexual? Oh yes, they are (quite rightly) supposed to behave professionally, deal with it and maintain the fitting professional distance. No one blames the boys.
B) It falsely singles out male teachers as victims. Straight female and gay male teachers as above face similar issues. And what about gay female teachers faced with make-up and short skirts? Oh no, they are not helpless victims either. Just the poor men.
C) Not only is it wrong in isolation, for the above reasons, it's also even worse because it springs straight from the kneejerk daily mail / misogynist tradition that women are responsible for men's lack of control - a message that infiltrates girls' and boys' lives constantly, and contributes to things like DV. So if the school have something to say to a hall full of y10 girls, it should be something that sends the OPPOSITE message, not something that backs it up.
How about spending the time telling girls they are equal in relationships, don't have to have sex if they don't want to, and can do anything boys can do.
Re uniform, they just point to the rules and say to boys AND girls, "these are the rules, follow them".