This isn't a traditional drying rack, but I was given a cafeteria cart
(the type you slide the plastic trays into) and it makes the best
drying rack, ever. Smaller art (clay pieces, jewelry, small paper
paintings) can go on the trays, bigger canvases can slide into the
slots, or you could take all the trays out and hang things up (like
tie-dye t-shirts or silks). You can roll it out of the way to keep
other kids out of the wet artworks.
Mine was free when they renovated the cafeteria, but I bet you could
get one donated from a hospital. Also, you might want to check
federal surplus warehouses. I got a lot of really good "industrial"
furniture there when I worked in public school. (I miss that!)