If you want acredididation then go get a art degree at a university. If you want to get experionts as a art director go pick up a book on how to design a game and down load unity. Lock your self in your room for the next 6to month and work a side job with enough pay to get by.
Now make your game and you now know what it is to be a art director. The point I am trying at is that a art director needs to have good communication skills, organization of tasks, be able to speak 'art' aka art skills, and know what goes into making a game. This is why college don't teach this because its a vary broad job to do and in turn require a vary large spectrum of skills.