One possible exception. I am NOT a lawyer. However it would not surprise me at all if it would have been perfectly legal for a parent to invite the Headmaster to slipper their son for some perceived offence, not in his official capacity as Headmaster but as a trusted family friend. Clearly if this was done the Headmaster would have been wise to obtain and retain some proof of the parental request!
This might even have been legal for an offence at school, if the parent felt the boy should be punished for it.