Remember, you are teaching how to recover from stalls, not necessarily how to stall an aircraft.
Power off stalls are supposed to represent an aircraft on final, landing configuration. Imagine being slow, dirty and never adding power and starting to come up short. Pilot starts to panic and pulls back on the elevator inducing a stall.
Power on stalls are meant to represent a stall on take-off. Either trim could be set too far aft (which would be a trim stall) or something makes the pilot panic on take off and yank the controls aft, inducing a stall.
So with that, one of the way private pilots wad up a plane is base to final stall/spin. Pilot is dirty and slow, shoots through final and tries to help the turn by not only making an aggressive turn, but adding too much rudder. AOA exceeds max while uncoordinated and plane enters a stall/spin.
Again, the purpose of doing stall training is to learn how to properly recover from a stall, not how to stall an aircraft.