The answer, no matter with whom one might be debating, lies in the severity of the injuries which were probably within the range of injuries deemed reasonable by some people in 1962 as far as parental and scholastic chastisement went. And that is why the prosecuting authority would have been reluctant to expend public money and police time on a case that many people in 1962 would have deemed to be not worth bringing.
As for police corruption, it is a fact (almost) universally admitted that that without that which the police call ‘noble cause corruption’, our prisons would be closer to empty than to full. With the greatest respect to our police officers, they are and always have been as bent as 45p pieces.