I agree with Sandra that acrylic paint peels off fairly easily with a
palette knife. I get a student to do this when they finish early and
they actually enjoy it. The palettes that are harder to clean are the
ones where someone has painted with tempera, let it dry, painted with
acrylic, then tempera, etc. I've had to soak those and scrub with an SOS
pad. Bleach didn't seem to help at all.