put dish soap to pretreat with tempra paint. you could also suggest to the teacher that they put a couple spoonfulls of dishsoap directily into the paint bottles and that way its like a pretreatment with the stain. if the teacher will it makes it wash out the first wash.
It depends on the type of paint used. Some of the paints will not come out. Also, depending on the project they cannot add the dishsoap to the paint, it changes the consistency. They should be wearing paint shirts to paint anyway.