Not all slipperings were hard. Some were quite jovial affairs where the person getting “The Slipper” sometimes enjoyed it. Teachers developed reputations based on how hard they slippered and a minority of boys became regular recipients of “The Slipper” being unperturbed by the experience or even enjoying it in all but the severest cases. There was kudos attached to getting slippered a lot and taking it like a man and possibly a minority of boys enjoyed the male-bonding aspect as well as perhaps the hot bottom at the end.
When I was at secondary school the use of the slipper was confined to the PE department for misbehavior in the Gym or a breach of PE dress etc. Sometimes a PE teacher would see you breaking a school rule and he would if it was merited slipper you himself. Girls were also sometimes slippered during PE but not as often as the boys.