I think some of you are being naive - as has already been pointed out on here, pictures of little girls at play, accidentally and innocently exposing their knicks, DO make it onto the internet and DO attract the wrong sort of comments. As do pictures of kids on the beach running about starkers.
I am also of the generation where we had to do cross country runs, at secondary school, in navy blue knickers. It was humiliating. I am not aware of any cases of girls being perved at but it MUST have happened. Yes, its the perv's fault not the childs but an easy compromise is wearing shorts as an extra layer - and they dont have to be worn all day, can easily be popped on for lunch/play times as my DD does
i can see the reverse thread "IABU, school will not let DD wear shorts under her dress and I dont want the world seeing her knicks"
the comparison with length of school skirts does not work - if parents sign up to a school, they agree to stick to the rules. In many JOBS you can get disciplined for breaking a dress code, school is no different. I saw one girl in the dentists the other day, her skirt did not even cover her bum when she sat down.