The only time my gym let us wear leggings was one winter when the heater was struggling and it was 45°-50° in the gym. We stretched in our leggings and sweatshirts and then we were warmed up and took them off. The leggings only were allowed to stay on the coldest days if we wanted but I took them off. The gym technically has a no shorts policy but if a girl who is 10-11+ starts wearing them once in a while they assume it’s to hide a pad and ignore the shorts. I love just wearing a leotard because I have sensory issues and at the gym is one of the only places I can tolerate normal looking clothes but I think girls should be able to wear athletic leggings if they really want to!