When students learn to shade, they begin to turn shapes into forms. We now have to get away from line and I begin stressing the importance of defining edges with value and shadows.
I saw a project on School Arts years ago with something similar. The teacher had created some forms with paper and then had the kids use pastels to draw them. I thought this idea would work for a shading project. Aside from having to shade, they also have to draw the intersecting forms.
When I introduced this project to my kids their mouths dropped. Comments like "I can't do that Mrs.!" and "No way!" were heard over and over again. With some encouragement and practice they finally felt confident enough to try. They were very pleased with their results.