This is not true of any school I have worked in. Every school has rules about disrupting class with provocative clothing.
The source for most of this is the continued media pressure for women to use their bodies as currency and for boys to strut their stuff as a big man. Look at the ads on TV an the shows. The most popular ones have women attracting men though their sexuality. 
The schools try to fight against this but  there is only so much they can do.