Is it ok to put a student driver sign on the rear windshield of a car (in CA)?

Ok so my son is learning to drive and is very slow…. no offense, but is it ok to print out a small sign that says student driver on it and put it at the very right of the back windshield? I dont have magnets or stickers and dont have time to buy them or pick them up? Note im in California.
no he wants the sign too…. is it legal?

great idea and could save him from experiencing road rage against him. i would check local dmv to see if they are looking at a program like this because here in georgia they are but till then check out this site.

http://www.rookiedriver.net/

Printing Magnum Magnetics DigiMag VINYL magnet on a Mimaki JV3-160S solvent inkjet printer