The Slipper was an informal, ad hoc punishment, used by ordinary teachers for classroom misbehavior. Ordinary teachers were not allowed to use The Cane: this was reserved for the headmaster and was very rare, disgraceful, and awesome.
I imagine the teachers who used the slipper were breaking the rules; I’m sure when the LEA wrote out guidelines that said “only the headmaster can use the cane” they thought that they were banning all CP outside of his Study. How little did they know?
The slipper was mainly used on younger kids 1st – 3rd years (11 -14) simply because the bigger ones would probably not have bent over when told to. (Bigger lads were sent to the headmaster, but I think that 5th formers were generally caned on the hands.)