I found that the custodians had some type of cleaner (which was in an
unlabeled barrel)....we called it the "blue stuff".....worked great on
everything......I didn't let the kids use it....but put it in a jar and let
the hard brushes soak.....everything from acrylics, to india ink came
off......see if your custodians have anything....sometimes they have some
good 'stuff' that you can "borrow"......