Power. Too many people aggressively cut the power on final. When learning (as so eloquently put by oneinsixty) we're not aiming for precision at the beginning. Demonstrate how (and when) they should stop looking at the numbers/instruments and should now be focussed on the horizon. At this point they should commence pitching up gently to the level attitude (the one they have learnt for straight and level flight) and then arrest the inevitable sink with balanced application of more elevator together with reduction in residual power. Take the lesson to the extreme and demonstrate a landing
maintaining the power setting you had on final approach.
If you try and get them to kill power on short final they will invariably get the timing wrong, the nose will drop (cows will get bigger
), they will overreact with elevator and the aircraft will drop onto the ground. There is a place for glide approaches but this isn't at the early stages of Ex 13.