We were wearing shorts under our skirts long before mobile phones where a thing, much less camera phones.
Because select nasty boys, and girls, would pull them up as a form of humiliation light hearted banter...
For the boys there was ‘debagging’...this was running up behind a boy and pulling down his trousers and pants. This tended to be carried out almost exclusively by select boys, but I remember the nastiest girl in our year doing it once, much to the delight of a the boys. 
You had to laugh at the humiliation light hearted banter you received because experience taught that to do anything else was to single yourself out as a target and become the butt of much more regular abuse gentle ribbing...
I actually felt worse for the boys in this cruel game, it somehow seemed more humiliating to have to bend over and collect your dignity than it was to have it fall back down after having dark cycling shorts exposed. A quick smooth of the skirt and it was over.