A Boarding school caning, something that I felt personally, is severe enough in its own rights and is a strong contender here but what might run against it is the six-stroke maximum albeit twelve is permissible with special consent from the school board of governors. Neither was allowed on the bare bottom but had it been it would most certainly be up there with a Reformatory caning.
A GRAMMAR SCHOOL caning I would imagine is the least severe here but on the contrary, I have heard tales of some very hard and severe canings being dealt out but, not unlike most personal accounts of heroism.