How should I hang a bent whiteboard?
Put long wood boards about 2-3" wide across the bend, and glue and screw the whiteboard to the 3" wide boards to force the whiteboard to be flat. Squish the bent whiteboard against the 3" boards. I prefer to add screws because glue often fails in damp environments, or just over time. 

Example: Curved whiteboard is on the left, straightening board on the right.