I wrote: The girls' locker room is clean. I was told I had a type, which I did not. Plural possessive has an apostrophe after the plural s. This type of sentence has been marked incorrect in several places. This error should be remedied.
It's accepted now.
In England the locker room is a changing room.
It still does accept girls' locker rooms. "locker room of the girls" sounds really awkward in english. English speakers generally do not form most possessive's this way.
I wrote "the girl's locker room is clean", which is how native speaker would say it, but I could understand if it should be girls', because the locker room would say Girls on the door.