I've started teaching a class on acrylics but they have only used poster paint in the paste. Frankly I don't remember using tempera paints except grade school.
I cut an 18-hole egg carton in half and used the bottom wells covered with a wet piece of paper towel.  This I fit nicely into disposable plastic squares with covers. I filled each hole with a few colors almost half-way and put their names on the plastic boxes.  Each week we give the paint a spray of water. They have not dried but once opened the children complain of a moldy smell.  I just smell paint myself?  I don't see mold but I did read somewhere if I add a small amount of bleach to the water it will prevent mold.  For the past 3-4 weeks this has worked for us but I am concerned that I do not smell what the children are smelling.  Any advice?

Professional silk screeners using acrylic based paints store it in closed contaners with ammonia. Drop the bleach. They both smell but ammonia is best. Don't mix ammonia and bleach, it makes a poisonious gas. Start over.

Thanks I'll try that...I haven't used the bleach yet. Do I simply add it on cotton in the box or the paint itself?

Pour some in the jar of paint, pour it out when you use the paint. It won't hurt if some gets mixed in.