When a mixed secondary modern (50s-70s) was a spanking school, the kids tended to get far worse at home. We experience everything in context. We thus make relative judgments. In a spanking school, CP isn’t a (relatively) light dose of the slipper from the class teacher, but the cane from the Head. Thus, in the mind of such a pupil, CP remains relatively rare.
For the women that got CP, did they say that their school was a ‘spanking’ school where a significant proportion of female pupils could expect to be punished this way sometime during their secondary education? I would suggest that a mixed secondary modern could be categorized this way if 30% plus of the girls were given CP sometime during their time there. Indeed, the best question (ideally) is what, in their opinion, was the apparent rate or likelihood of a girl being punished at some time in their school? This would assume classroom punishments, which either existed in a given school or did not.