I teach high school and solved the classroom cleaning problem by allowing
students to own their supplies. They pay an activity fee each year which
covers the cost of art supplies. At the beginning of each year, students
receive whatever pencils, brushes, paints, colored pencils, markers, etc.
they will use for that year's projects. They are assigned baskets in which
to keep their supplies, and mark everything with tape.
I never have to wash brushes anymore, and at the end of four years,
students have a very nice collection of supplies.