You raise an interesting issue, but clearly it isn’t the one you meant to raise! You are indeed correct, in my experience at least, in saying that punishment books never show ‘six of the best’. This I think is because punishment books (at least those I’ve seen) show the number of strokes administered. They don’t often show the severity of those strokes, and even if they do give some indication, in a remarks column etc., they almost certainly don’t say ‘of the best’. Any remarks on severity are likely to be somewhat more guarded. Teachers, very sensibly, are not in the business of providing unnecessary hostages to fortune, and a lawyer could make considerable capital out of a term like ‘of the best’.