You first claimed that of course girls weren’t showing their arses (some definitely are) and now you’re claiming that it’s not a problem if they do (it is).
It’s my workplace. I don’t want to be seeing arses in my workplace. Other workplaces have dress codes and men and women would get pulled up for not following it. It is not my problem for thinking that people should wear appropriate attire in schools. Teachers, similarly should not turn up with their arses on display.
You’re trying to make dress codes about misogyny. I don’t want men turning up with no shirt on either, but that comes up far less often. Appropriate attire for both sexes, please.