We learnt in tandem seating aircraft, instructor sat in the rear, one student had his instructor undo his straps, lean forward and smack him about the head with his knee board, it's why we wore helmets, student wondered if he could get away with ramming in a bunch of forward stick but thought better of it.
A story circulated at the time about a previous instructor who had the reputation of being a screamer, always regarded the story as being just that until I happened upon these videos.