A variation on Darren's comment;
Buy some thicker stock styrene, I'd say no less than .040" gauge and snug it to the underside of the warped piece with c-clamps (use some paper towel between the clamps and the plastic to avoid scuffing the plastic. Once the pieces are snugged together, the warped piece should be generally flattened out and you can run a line of super glue around the junction of the warped part and the stock sheet, leave the clamps on overnight for fullest curing of the glue.
As your piece is a flat ship deck, that should solve your problem.
Good luck, hope your son gets a good mark on his report.