One of my first purchases as an art teacher were these wooden, geometric forms. I use them every year. I like to teach my students precise methods of using their pencils to create smooth marks without smearing then we pull out these forms. I demonstrate and we cover shadows and shading for each of the basic forms. Then the students have to combine them and create a composition.
I have extension cords running from all over the room so each group of students has a tray with their own light source. I've done this type of activity since I first started teaching. I imagine every art teacher out there has something similar in their drawing classes.
My classes are currently large - 27 or so in each of my beginning classes. I am fortunate to have a big classroom which can accommodate all these people and have room to store their work and all my supplies.