If the teacher really wanted a girl to cry, he could have exploited the embarrassment factor (I prefer the word “embarrassment” to “humiliation”) which could be quite powerful when a girl was made to bend over for a slippering in front of her classmates, which may sometimes have included leering and jeering boys. Of course the kind of teachers who used CP were usually those who would never tolerate such jeering, but maybe that was because they didn’t know much about psychology.
On the other hand, a girl being slippered in front of such an unruly type of class might sometimes often be determined not to cry because to do so would be to give the jeerers something extra to jeer about, so they might just grit their teeth and put on a brave face.