To return to the original subject, and Corncrake's evidence:
corncrake:
Navy blue gym knickers were always worn as a standard part of the school uniform
Does this mean gym knickers were worn for all lessons ie, "gym knickers" simply meant "school uniform knickers", not particularly associated with PE? If so, that's quite different from what others have said. Possibly a regional variation or English/Scottish divide? (I believe PinkAngel was educated in England)