I was quite shocked the first time I was slippered at grammar school as I hadn’t expected it to hurt that much. It was only two smacks as well.
I did get two token slipperings at junior school from a female teacher but these were very soft smacks. What hurt more was a spanking from a male teacher who lined up about six of us and gave us each quite a hard spanking, probably about ten smacks. It was nothing like the pain of the slipper though.