From reports both here and elsewhere, it seems whole class punishments (or half-class in the case of most mixed sex schools) were not all that uncommon. In the case of the only one I was involved in, administered by the PE mistress standing in for our English teacher, we were all messing about in class so none of us was slippered unfairly – but it could easily have happened.
PE teachers did seem to have something of a reputation for being “slipper happy.” In other lessons, forgetting or failing to do homework most often resulted in an instruction to “bring it in tomorrow”. Minor misbehaviour in class would usually, initially, elicit an simple warning – often just the pupil’s name being spoken by the teacher. In PE, forgetting (admittedly sometimes intentionally) gym kit or misbehaving in any way usually meant an immediate meeting with the slipper.