There might have been half a dozen canings a year throughout the school, whereas the slipper must have been used half a dozen times in class during my first fortnight of junior school, and that was just one class out of eight in the school. (I admit that our teacher that term wielded the slipper more often than most.)
In terms of the atmosphere in the school, the widespread prevalence of classroom CP very much overshadowed the occasional use of formal, more severe, CP, but would not register in the historical records at all.