The girls feel quite angry as well but school refuse to even have a grown up conversation with them. In my opinion to avoid being called out on it.
I suspect this is the problem. The policy is deliberately vague. Perhaps they are uncomfortable getting into specifics. Because the focus will then be on why is a short skirt wrong? Why is a strappy top wrong? Cos you know, teenagers will ask.
Unfortunately, it's backfired anyway, simply because it is too vague.
OP, although I understand your daughter wanting to make a point, from a parent's point of view, I'm a firm believer in picking your battles.
Let her write a sensible, mature email pointing out that the vagueness of the policy is leading to difficulties in interpretation. She can also make a point of highlighting that the policy should apply to both sexes.
Meanwhile suggest to her that she adheres to what she suspects is the policy goal.
Hopefully she will get a sensible reply, and it might force the school to take a more specific line with what they have in mind. And why.
The vagueness is the annoying part. The school should have the courage of its convictions and be able to defend them adequately.