Circular saw, plywood blade and cut it from the back side. put a piece of masking tape over the whiteboard cut line to control splintering.

If its plastic coated ply a high tooth count blade, if its epoxy or colourbond coated metal you will need a thin sheet metal blade like Freud make. In both instances cut from the back and cover the cut line on the writing side with masking tape.
Circular saw, plywood blade and cut it from the back side. put a piece of masking tape over the whiteboard cut line to control splintering.