I used 3/4 inch plywood. I had it cut large enough, all around for a frame
and for grout in-between the tiles. I put the empty plywood on the wall
before attaching the tiles. I used iron nails and a nail gun to attach the
plywood to the wall.
A local tile company came to help our fourth graders. The two men who came
had been students of mine several years back. They used Mastic to attach the
tiles to the wood. Then a few days later came back and with the fourth
graders, grouted.
We then spent several days washing the grout.... and the floor.
Then the kids painted the frame with water-based enamel. They chose the
color. Not the color I would have chosen, but they were right. It looks great.