In my catholic boys’ school in the 70s, the slipper was used extensively. It was always a plimsoll, large and flexible, and there was probably one available in every classroom. Often several boys in the class would be slippered in one day.
Different teachers had different routines. Some would administer the slippering at the end of the lesson or at break time – so you had the dreaded wait, others would tell you to step outside, leave you a couple of minutes then appear with the slipper to dish out the punishment.
Most commonly the slippering was in front of the class, usually with your bottom facing the class.