“Six of the best” came up at least once in virtually every such story, and here means a caning on the bottom of, mostly typically, a secondary-level boy. This notion was certainly alive and well at my ancient grammar school in the 1960s and I should be surprised if that owed anything to 19th-century school board regulations.
The question really then is, where did the public schools and the grammar schools get the figure of six from? So much of their ethos — including, indeed, the whole idea of whacking teenage boys on their bottoms — seems to have been derived from classical studies; could it have its origin in Roman times?