PE teachers were the cause of most lads getting slippered at some point in school (mid to late 70s) but part of that I am sure was down to the fact they were in a less controlled environment and with lads often out of direct view unlike in a classroom and needed to establish just what the limits were.(Craft/woodwork/metalwork teachers could be known for using the slipper quickly too) As George says seeing a lad or lads get slippered there and then as PE tended to do soon made the rest of us toe the line and behave.