I approach shading like the photographers Ansel Adams and Edward Weston did, through the Zone system. I have each student do a value scale (10 steps from white to black). They keep the value scale and when we do our first drawing they are told that all 10 values must be evident in their finished drawings. I then give them a lightbulb which they place on a white piece of paper. They draw the lightbulb, and 'shade' it using all 10 values. As for the "act of shading" I do not allow my students to smear with their finger, or use a tortillion stump, they must manipulate their pencils on the paper for the values, alone. As a class we talk about light sources on the lightbulbs that they each have in front of them. Then they are charged to draw the lightbulb. I go around and they hear me say that they can not shade until their drawing is finished. Once they have established the drawing then they are charged to look for the "lightest lights, and the darkest darks" and carefully render the values.