France recently banned the Abaya from schools which is just a long dress.
If only short dresses are allowed in schools because "we need to ban religion" then following this exact logic they could just ban all dresses next.
Following that just make all girls go to school in bikinis because "religious people wear clothes".
In Africa there's tribes with women who aren't even wearing anything on their chest because that's where those women believe the line should be. From a secular point of view would you also accept it if teenage girls started going to school without clothes?