Moody:
Back in the 1970ies most items were packed in dozens.
I believe Moody might have the correct idea.
In my younger days, I often heard the phrase 'six of the best' or 'six of the very best', or even a 'dozen of the finest' applied to everything from apples to pastries to eggs. I would suggest that the origin lies in normal discourse in shopping and was only later applied to school CP when it was only ever used when referring to six or twelve. In school we never referred to 'two of the best', 'four of the best' or even 'eight of the best' even though four or two strokes were much more common. However if the punishment was to be six then the phrase 'six of the best' was used and not just by the teacher, we children would also talk of 'six of best' .
If it was a phrase that originated in relation to CP then , logically, phrase such as 'four of the best' or 'eight of the best' would also have been used. As I said, my memory is that four, or even sometimes only two strokes was much more common, six or eight being exceptional, at least in my schooldays.