I was at secondary school from 1969 until 1976. It was an all girls school and leg slapping was still very much a favoured punishment.
For girls up to and including the fifth form (ie aged 16) white knee socks rather than tights were required to be worn, and this made it very easy for a teacher to get her hand to the backs of bare thighs.
The system was for girls to be called to the front of the class, and then turned around to face the class whilst their legs were slapped. The class was therefore obliged to watch a classmate’s wincing, grimacing face as the sting built up in her legs.
I can recall trying desperately not to shame myself by crying in front of the class but that was simply impossible for me – and for all the girls. I’m sure in fact that if a girl seemed like she might be able to hold back the tears, the slaps would get harder and harder until the tears began to flow.