Make a quilt. Encourage the children in the class to bring in a cloth square of a specified size which they have decorated. The children can paint on the square, embroider it, or employ any method they like. During the school day, the children can work together sewing the squares into a quilt. Display the completed quilt in a public area inside the school, and auction it off.
Work together as a class to design and decorate a kite. The kite does not need to fly, necessarily, but can be a work of art. The kite can be auctioned with the proceeds going toward the school.
Create a mosaic from tiles the children decorate themselves. With supervision, the children can create a frame or turn the mosaic into a table top. Auction the mosaic and put the proceeds toward the school.
Buy inexpensive aprons for the class and invite the children to decorate them. These aprons don't necessarily need to have similar designs, but can be as different as the individual children in the class. Auction the aprons and put the proceeds toward the schools.