The rules: No new paint. Only leftover paint and geometric shapes.
I gessoed the boards first. (We painted on cardboard scavenged from one of my school's recycling. These are 22 1/2x 28 1/2- the boxes that were used to ship railroad board.)
I started each with a shape in a corner- part of a circle or a rectangle, etc. and asked the artists to build on that. As they finished painting their paper mache, they could come work on the group paintings, adding shapes, layering colors and patterns.
Make it a Wonderful Day!