I feel I must correct this. The use of a smooth slipper was described as “fairer on the boy’s bottom.” I think this is rather misleading. The reason teachers used a smooth sole slipper was that a greater part of the sole made contact with the boy’s bottom, rather than just the tread, and this made it sting more. It was not as if a shoe with tread marked the bottom any more than a smooth one.
There was great diversity in slippers/shoes that were used, as unlike the cane it was an unofficial punishment, and the implement did not have to be approved.