It wasn’t that vicious; you would sort of swallow, rub yourself, and go back to your seat, and probably have recovered by the end of the lesson. Someone said it was like hitting your thumb with a hammer; you didn’t want it to happen but if it did it didn’t spoil your day.
There wasn’t a rule that said that girls couldn’t be slippered, but I can’t think of a specific instance of it happening, maybe because girls were better behaved. (I can remember a couple of times that girls were reported to have been caned by the head, though…)