Maybe on types where the door is liable to pop open in flight the instructor should make sure the student has seen it at least once during training ... ?
When the door popped open on my first ever flight with a passenger I just explained to the passenger that this was no big deal ("it's the seat belt that stops you falling out, the door is just to keep the draught off") and dealt with it. I might have coped less well if it hadn't happened to me during training.