I would go with white trim and colors on the walls. A sage green, taupe or a pale gray blue are likable colors by most. Then go for colorful bulletin boards and book cases. In colors like Aqua, Orange or red would go with any of the wall colors I just mentioned.

Blackboard paint in bright colours can be used in unusual places - and the children might well be amused that the teacher is 'writing on the walls' will allow you to keep things changing ! I up cycled a desk with a totally chalk board top to doodle on - that was interesting - used it on dining tables in a cafe too ! Also I would suggest keep base colour calming - green as already suggested is good ! You don't want the kids too hyper !!!

I am not a decorator, but I am an elementary school principal. PLEASE do NOT paint the small classroom a dark color. I'd go with a light neutral - one of the 'shades of white' on the sample cards. Then, as Kevin suggested, allow the color to come in on the bulletin boards. There's nothing more depressing than a small, dark classroom.

Are you talking about public and private school classrooms?