How do I know what code to use for box 12?

My W-2 form says "See Instrs. (instructions) for Box 12" in box 12.  On the back, it lists and explains all the different codes.  I have no idea which one I'm supposed to use or why the employer didn't fill in box 12 themselves.  This W-2 is for a summer program that was held at the school I teach at, but run by a different organization (hence the separate W-2).  I only worked there for about 15 hours one week to fill in for someone.  Wages only amounted to 564.00.  What code should I use?