It might or might not make a difference if the punishment was carried out on school premises in school time, but my suspicion is that any offence in that case would have concerned the misuse of official time and facilities rather than the actual slippering, and would have been contractual rather than a criminal offence.
I suspect that however undesirable it might have been, such an arrangement might have been possible between parents and Headmaster (or rather ‘family friend’ in this context) until the law governing parental chastisement and other than very temporary marking came in. I wouldn’t have thought it would be possible to use a slipper with any sort of effect without exceeding the limitations of that legislation.