“We” actually told everyone what NOT to dress, because some dresses are actually not (only) garments but religious symbols. Again, if you use this argument I will play the devil’s advocate and support people going in KKK uniform to school. Wouldn’t we want to tell boys how to dress, no? Or a good ol’ SS uniform.
Clothes sometimes are more than pieces of cloth we cover ourselves with, and some of them have religious value, whether you acknowledge it or not. You can argue that for you schools should NOT be a neutral space (regarding religion), but you can’t make up argument such as religious clothing being worn for modesty.