I do think most teachers took care to select a slipper that was suitable for its purpose. Having said that for a short while some teachers used large Green Flash plimsolls that had thick, chunky rubber soles. These tended to leave a bruise in the shape of the sipper on the boy’s bottom.
One thing needs noting. Most PE staff just used the size slipper that was handy at the time not one kept in a special place just for the purpose. It is also worth noting that they did it on the boy’s shorts, and as pants were not allowed in most schools for PE/games the sting was much more intense.