Chalk dust method
The chalk dust method is said to have been used by some teachers in school corporal punishment. In this method, the teacher would make the delinquent student come to the front of the classroom and assume the position. He would take the chalk eraser (the cloth or sponge that was used for cleaning the blackboard) and use it to wipe a little chalk dust onto the seat of the student's trousers or shorts – or the young missy's skirt or dress. Then he would administer the caning. The method mimicked the way canes and carpet beaters were used to dust clothing off – but with the student inside. The punishment went on until the student's seat was satisfactorily cleaned this way.
What may appear at first like a bad joke had probably a sensible, practical side to it. The method may have been used to introduce greater fairness, since the number of strokes can be a poor measure for the severity of the punishment (as a stroke can be softer or harder than it seems to the teacher). The chalk dust gave a roughly objective measure of how much the student's seat had suffered and when the punishment was enough.
Different levels of punishment, determined by the student's offense and age, could be measured out in advance by the amount of chalk applied.
The chalk dust method may have also helped teachers who were somewhat uneasy with administering corporal punishment, as it is psychologically easier to administer a caning as a "cleaning job".
Another variation as described by Roald Dahl's childhood account Boy was to apply the chalk to the implement such that when the cane struck the child's seat, it would leave a mark where it hit. This was a method to keep track of where the strokes had landed, and was viewed as a trick of a 'lesser' skilled caner. In the film Flirting, the Housemaster applies chalk to the cane.