While it was common for us boys to be slippered at my mixed school it was unusual for girls to receive the slipper. Those that were was mostly in PE. Outside of PE, it was very rare. Those that were received the slipper across their skirted bottom or the cane across the hand.
At the boarding school I attended the slipper was only used privately, mostly in the Dormitory Mistress’s room. Usually over knickers, but these could be lowered if the situation required.
During actual lessons in school, no girls were ever slippered in front of the class. I had a few different P.E. mistresses over the years and none used the slipper. Nearly all went for the bare upper thighs, but only with the open hand. This was very common and even occurred in upper secondary.