I used baby wipes for hand and table clean up.
For water, maybe you can get one of those portable water bottle setups and
get the custodian help you keep it filled?
For dirty water, I used to keep an empty bucket on my cart and would empty it
into the custodians sink after class. Is there one nearby for you? Sounds
like getting rid of the dirty water is going to be harder to manage than
getting clean. Is there a drain nearby you could use? Temper paint and
watercolors are non toxic and could probably be dumped outside. Im not sure
about acrylic. Good luck with your situation. I'm sure with your creative
mind, you'll figure something out. Janet