There is actually a case that enforcing these rules leads to better behaviour in the classrooms. Kids will push the boundaries wherever they can, we did it at that age too, and where the school is strict on uniform and the kids know they can't get away with anything you can find that this...
The actual problem is one of girls rolling their skirts up to make them shorter. Parents will send them out with the correct skirt then half way down the road they'll get rolled up. Same in school, teacher in class 1 will tell them to roll skirts down, walking to class 2 skirts rolled up again...