I don't recall the details but a good friend did something similar with his daughter's elementary school class. He brought in a banner sized piece of canvas. It was coated and the children were asked to bring in items and they did a large photogram outside (which also included themselves). The banner was then hung permanently in the school.
My kids are now of school age, so I hope to do something like this. It would be to inspire them with the magic of "old school" photography.