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This may or may not be feasible for your situation, but I used to have
the same problem with my HS students. It wasn't until I gave them
temporary ownership of the brushes (with their initials on a masking tape
"flag" on the handle) that they started taking much better care of them.
We do the same with palettes, palette knives, drawing pens...anything
that needs a thorough cleaning at the end of class.
Hope this helps.