The best way I found to get acrylic paint out of brushes is to not rely on kids to wash them! There is simply no way to visually tell if the acrylic is all washed out of a brush, which is exaclty how kids approach most cleaning tasks. Therefore, I used to set up a large plastic container with soapy warm water in it, and have the kids put the brushes (bristles down, of course) in the water to soak. Once class was over, I made sure I scrubbed them out myself, usually a simple task, until the brushes rinsed clean. I used a variety of brushes, mostly camel hair in different sizes, for painting. On plaster work, which requires a stiffer brush, I liked to use those "chubby" brushes, which have hog bristle instead of the softer hair used in camel hair brushes (which aren't really made of camel, by the way).