I think we were told about it in order to impress on us exactly what we could expect if we ever dreamed about doing such a thing – but that was the only occasion I really remember being sure that someone had got the slipper.
When I was spanked at home, if it was Dad, it was always with the slipper or his hand or sometimes both. Neither of our parents actually possessed such a thing as an actual pair of slippers, so in reality it was a leather sandal with a smooth rubber sole. He held the heel end and walloped with the sole part. It was always administered bare bottom (I doubt the ones at school were) and over the knee in our own bedrooms. It made a very noisy smacking sound and stung like hell!