First off, every school i have worked at in NY and GA have had dress codes. It is a little harder for male teachers to enforce it on females, but at the school i am leaving at the end of this week (to move back to Buffalo) the rule is that the girls' skirts must come within 2 inches of the knees no skin above the waist and below the chest may be exposed, and the breasts must be completely covered. So no spaghetti straps, miniskirts, short shirts, etc....
That said about the only one of those rules i choose to enforce on a regular basis is the skin about waist one, just cuz it is easily noticable, and on half of them i really don't want to see their thongs. The only times i have had to enforce the chest rules was one with giant chest was wearing a very low cut shirt where one of them would have popped loose. And another the girl had a kittie kat on the shirt with buttons where located in about the region of the nipples, so the girl kept playing with buttons.