Yes of course lost property is the most likely answer. The school is donkeys years old so I expect they had all sort of things accumulated over the years.
I am quite amused by the idea of those teachers wishing to arm themselves with a `slipper` having a root around in lost property to find the most effective
They would have wished to have their own on hand, they could not rely on a willing pupil to come up with one. The wearing of `pumps` in school was frowned upon and as far as I can remember we only brought PE kit to school on days we were scheduled to do PE. The teacher would have most likely come unstuck with a request for a slipper.
When they were required to give up their slippers I wonder if they binned them or returned them to lost property.
Actually they are probably still languishing in the desk drawer!