Crisp iron shots require a slight descending angle of approach to the golf ball.
Students need to understand that the golf swing is a circle understand where the bottom of the arc of the circle should occur (slightly in front of the golf ball). The question then becomes how to move the bottom of the arc of the circle? Weight shift is the correct answer!
My favorite drill is to paint a line on the ground and have students try to hit the ground or grass in front (side closest to target) of the line.