The girls I know who were slippered at school received it from the Headmistress and they all were made to wait until it was convenient.
All the said Headmistresses kept their own slipper a plimsoll, except the Headmistress who gave my wife the slipper and she told me that was a mule, from my wife’s description though it was more like a pump.
I expect a Gym Mistress wouldn’t need to keep a slipper, she would have one with her at most times.