The teacher would then have told the class to look to the front and pay attention, but throughout the remainder of that lesson, the other pupils would have made further quick glances at the girl whenever the teacher’s back was turned.
The boys in that class didn’t need to see that slipper landing on that girl’s bottom to become sexually aroused by what they had seen & heard that day. Later, when running through those events in their minds, they would have had no difficulty in using their imaginations to create that one missing piece from the jigsaw, and that would help to prolong their thoughts and thus make them more indelible.