I got the slipper plenty of times at primary school, but never secondary, and I can safely say that it varied from teacher, at least at my school. The PE master had a monster size of 14 feet, and he kept one of his old ones handy – it was pretty awful. The other masters usually had one from lost property – ranging from my very first teacher who kept a very feeble, tiny little one which must have only been a size 1 to my final teacher there, who had a size 9 one left behind by some mammoth child of previous years.