“I don’t know if our Headmaster had a kink but it seems strange that we received some canings over our underpants”
I expect your Headmaster was a true professional and had your best interest at heart. He knew that the caning would only do you some good if it was painful and he may have had a policy of one layer of clothing to achieve that aim. It also enabled him to make sure you didn’t have any blotting paper stuffed down your trousers.