I use a wired on 1/4" plywood rack extender for big, heavy loads such as bags of cement.
For a lighter school bag, a few layers of coroplast glued, bolted, rivetted, ziptied together will be fine. Punch some holes, add some reinforcement and ziptie to the rack. This should last a couple of years.

The materials are zero cost, you can pick up coroplast and zipties for free.