I was fortunate enough to have an artist-in-residence complete a mosaic
mural at my school last spring and I took lots of notes.
There is a product called wheeti board (phonetic spelling) which is like
sheet rock but reinforced with a plastic mesh and it can be cut to any
shape with a sheet rock knife. It does not warp and is very lightweight -
lighter than sheet rock. I don't know if it weatherproof, but you could ask
at Home Depot, that is where the artist picked up the product. Local tile
stores donated tiles and I'm sure parents would be willing to donate.
(Another school I taught at previously had a parent who owned a tile
company - jackpot!) The wheeti board was tiled and then mounted with screw
bolts to the wall by our maintenance crew. I strongly suggest goggles when
breaking the tiles and a canvas tarp that you can place the tile in-between
so no shards fly out. Parent helpers are always an added bonus. Also, the
artist gave the kids wood skewers broken in half to apply the mastic onto
the back of the tile so there wasn't gobs of mastic on the back of the tile.
I don't know if that is necessary, but it was something I wrote down. The
kids wore plastic gloves (non-latex) and took their shoes off because it's
messy. So much fun!
Sounds like a great plan - good luck with the project.