The system was that you had to stand up against the desk in that room, hands on the desk. The Deputy Headmistress would then pull a seat up, sitting down at your left so that she could inch your skirt up a little with her left hand and then slap with her right. Shed start just above the knee on one leg and slap right the way up your thigh, then repeating this several times before turning her attention to the other leg.
She intended the slaps to hurt and they truly did. It was also hugely embarrassing as a teen to be smacked that way and Im ashamed to say that I did a lot of crying on both occasions.