The Headmaster quoted should have been reported to the School Governors by the staff who were aware of his practices. If that failed to secure action they should have raised the matter with their Union, because they were all at some risk, if only of being judged professionally incompetent. Had the parents of boys slippered complained the Headmaster would undoubtedly have faced some sort of sanction, though it might have stopped short of legal action.
If the boys slippered were to complain now and could establish the truth of their claim I would assume they could have legal action of some sort initiated. What the outcome of it might be I have no idea, and indeed it might well be that the CPS would decline to take action for that particular offence after so much time has elapsed.