When we are having brush washing issues I do three things- 1. Start cleanup five minutes sooner 2. Require all washed brushes come to ME for check 3. Worst case scenario Stop using Acrylic paint. I make it very clear I am not required to teach painting with acrylics, it can go away if not treated with respect.
I keep a jar of primo acrylic brush cleaner and stick in any unwashed brushes I find until I have a batch to really clean.
I do not hesitate to tape a dried brush onto the artwork in the matching color and class period. I do not mind the time it takes because it Works like a charm.
If you have a student store you might consider selling a really nice brush there, like in a different color than what you have in your room -- so wasteful kids with money to burn can purchase and then abandon them everywhere for you and other kids to rescue. I'm considering this myself.