Corporal Punishment & Slipper Sizes3

When in Sec Mod it was usually a size 9.
Strangely we had a little ditty at one time, it went “You get more whacks from a size 9 slipper to whacks more” etc sung to the tune of the Knorr soup advert ( more soup from a packet of Knorr” etc.
Now in hindsight where did all those worn shiny soled size 9s come from, most of us had smaller feet and in any case, I cannot see any of us donating them to the slipper fund. Also, what happened to the left-foot ones?
Was there some kind of specialty worn soled plimsoll supplier? Perhaps the Gym teacher wore them out quickly and auctioned them off?
Curiouser and curiouser as Alice would say or was it the Cheshire cat?
Thinking on the slippers seemed great big things especially considering the target size, unless of course, some particularly broad-beamed older girl got it