There's a reason we're spending a lot of time practicing seeing a subject as a series of layers. For watercolorists, it just may be the most useful skill of all.
Here's an exercise that makes the progression of layers very easy to recognize, and creates a rich resource for the class at the same time:
Find an image that readily resolves into 3 or 4 layers (light, middle, dark, super-dark).
Paint the first layer.
Get another piece of paper, and paint the first and second layers.
Get another piece of paper, and paint the first, second and third layers.
If there is a fourth layer, you'll need a new paper, with all 4 layers on it.
When you're done, you should have a series of pages, showing the step-by-step progression.
Imagine the classroom wall with all these arrays displayed (displays arrayed?)