An assignment passed down from the art teacher who came before me (she resigned half-way through the year I was hired, those 8th graders were rough), which was adapted from an art assignment she found online. I do not know her source, but after some digging, I believe it can be located here. I, myself, have made my own alternations to the assignment, namely going over the element of art, Space, in much more detail before starting.
The idea is for kids to understand depth and how to render it on a 2D surface.