Let me understand this -- how many squares of muslin does each classroom get? And how big are the squares?
You might want to check out the book entitled, "Handprint Quilts, Creating Children's Keepsakes with Paint and Fabric" by Marcia L. Layton. Instead of the traditional hand prints with names, her ideas include making designs with hand prints, including animals, flowers, hearts, etc. Some are embellished. In order to get contributions from lots of kids, you might want to consider a wide strip for each classroom to design -- her book shows flower gardens and other ideas for landscapes.
What a challenge!
Dayle
I am envisioning 400 kids possible with so many classrooms. You might need to have artwork on the backing in order to include everybody.