I teach watercolor workshops and drawing to beginning through advanced artists, and because of the wide range of skill levels, I had to develop a series of exercises that span those levels to keep everyone engaged. I think some of these techniques could help your situation.
First, I start with studio exercises geared to teach basic drawing or painting techniques in an unusual way.
1. Gumdrop still life: This is fun and edible at the same time! Each student chooses gumdrops of a variety of shapes and sizes and constructs a still life on a sheet of paper. This exercise teaches working with shape, mass, overlapping, texture and shadow.
2. Balloons: Give kids 3 or 4 balloons, and have them inflate them to differing degrees of size, some partially inflated and one fully inflated. Place on a large sheet of white paper in an arrangement and draw. Kids can tape them to the paper to keep them from moving. The balloons present the problem of drawing translucency, volume, mass and how shadows are effected by those elements.
3. Ribbons: Give kids 3 or 4, 6 inch pieces of shiny ribbon, and have them curl some and leave others alone. Place in an arrangement that shows overlapping and adjacent objects. This project teaches rendering shiny objects that have a variety of translucent to opaque qualities, reflective surfaces, and how objects relate to one another in a composition.
4. Glass of water with object: Give kids a glass 2/3 filled with water, and place a brush or pencil in the water, tall enough to stick out of the glass. This is arranged on a white sheet of paper and drawn. This exercise presents the problem of showing how water refracts and reflects an image and focuses on observation skills.
5. Two point perspective: I think it is a must for students to understand the basics of linear perspective. Set up a rectangular object on a sheet of paper and position it with the corner facing the student. Draw the object with a view to observing how converging lines work as well as shadows without showing the edges of the paper. Two point perspective is the most commonly used viewpoint in artwork, so I would focus on that first. I developed an easy to follow sheet of instructions to teach perspective that I have taught from 5th grade to adult with great success.
I hope some of this makes sense. Contact me off list if you need clarification.