The only problem is that Justin doesn’t know the identity of the boy, or his age. Indeed he has only told us that he knows of the incident from a conversation between his mother and a neighbour, that he heard when he was aged eight or nine.
Justin wrote “Prima facie the Headmaster’s action would appear to constitute GBH or ABH”. I always understood “prima facie” related to evidence that was not contested, but I read no evidence here, only hearsay.
It could be that the injury to the boy was accidental – that a cane stroke intended for the buttocks landed unintentionally on the leg. It does happen and at an awkward angle, the tip of the cane might dig in.