strapless prom dresses

I've never understood schools
"The Dress Code" policys.
Boys run around shirtless playing basketball.
Cheerleaders wear short skirts.
Gym class is shorts and then showers afterwards.
Girls wear thin or strapless dresses at Proms & formal dances.
I can see having rules against obvious provocative wear like daisy dukes or shirts that show cleavage... strapless prom dresses
But the rest? Give me a break.