My 11yo dd goes to a tap and ballet class, and has just had a letter home to say that although the leotard/skirt combination they currently wear is still fine for the classes, when they take their ballet exam they have to wear just the leotard and tights, so the examiner can see their legs.
My dd will be seeing AF within a couple of years. Her best friend already does. If they don't wear tampons its going to be glaringly obvious that they are having their AF, and if they're at all self-conscious you can forget them doing well in the exams.