During the warmer months, the girl’s uniform changed from a normal skirt & blouse to a thin summer dress.
Some girls wore these dresses very short; this was particularly true between 1971 and 1974. When such a girl had to bend over for the slipper, her dress would, unavoidably, ride up to reveal her knickers. In such situations, it was also possible to engineer the maximum possible upward movement of her dress by making her bend over far more than was the norm while in a semi-unsupported standing position (as opposed to over a desk or chair) and then placing one hands on her lower back, ostensibly to move her slightly to the left or right, but in truth to craftily slide her skirt a little further up. I never did this when slippering a girl in front of the class for fear that one of the other pupils might spot the trick. So whenever I expected to be presented with such an opportunity I would always leave the slippering until after the lesson when there would be no witnesses.