"You mean girls don't see OTHER GIRLS nude in the locker rooms and showers following gym classes or when working out at the rec centers?"
Oddly, in the vast majority of places in the U.S, no. Unlike in men's locker rooms, locker room showers in schools and gyms are in stalls with curtains where they can get undressed, shower, and redress without being seen nude - or seeing another woman's body. This is not the case everywhere, and I think it's rare in Canada, and not at all in Canadian schools (except for one or two showers with stalls, used by girls who are menstruating if they prefer).
In Japan, locker room shyness is foreign, and bodies are sometimes commented on matter-of-factly. European countries vary, but places like Germany and the Nothern countries more often than not have nude, shared saunas (usually separate locker rooms though, but again no shower or changing stalls).