I wonder if any members had the system in their school which was fairly common. That being a detention could be replaced with two of the slipper, often given by PE staff. I think this encouraged or allowed pe staff to abuse or administer unreasonable punishment. Boys that took these two whacks could in some schools be at risk. I always kept a close check on it as head, but I fear many heads did not. The view being if no complaints were received than everything must be in order.
The trouble of boys fell into three groups basicly fore this replacement punishment. Those that had other commitments. This was not an area of concern.
Boys that had received several detentions and were under a threat of death from parents. These boys would opt, and the pe teacher know it, for the slipper so their parents would not find out they were in trouble again.