It works in both directions, too. When I started (public) middle school in the early 90s, we weren't permitted to wear shorts. It was long pants for boys at all times and girls had to wear either pants or modest skirts/dresses (IIRC skorts were also okay). Keep in mind that this was Central Florida, where summer starts in March and ends in October.
At some point during my sixth grade year, the rule was changed and shorts were allowed. By the time I started seventh grade, some girls were already eschewing shirts for bikini tops. I'm no longer in the area, but I've heard from friends who still are that the '14-15 school year will be the first one where clothing is completely forbidden, though the orgies remain optional (for now).