SanD just mentioned mixing plates. This year I bought some of the new no-stick palettes for acrylic. I cut one up into smaller pieces and kept one big for several kids to share. We use acrylics and the paint honestly does Not stick. Kids were more willing to use paint mixed by the previous class, we never cleaned them, just kept adding, then at semesters end I peeled the paint off in large sheets, which are very awesome. I plan to cut letters out of it for my year opening bulletin board and maybe make book covers.
And best of all, all that paint didn't go in the trash!