Perhaps if you qualified why you consider Pokemon Red art, then she would have room to work with. I'm sure that your teacher gave clear examples as to what she considered appropriate subjects for art, and you really should've taken time after class to ask her if Pokemon Red was an acceptable subject or at the very least argue why it should be considered. If I wrote an essay about why I considered Chinese Checkers or Blackjack a work of art that influenced my life, I'd probably get a few uneasy looks myself.
So: Next time make the effort to explain why you thought Pokemon Red should be considered art, OR choose another video game that would be easier to demonstrate the artistic merits of video games to a broader audience (I'm talking about your Okami, Ico, Shadow of the Colossus, and Silent Hill 2 games here).
Pick your battles carefully. You'll have an easier time diving into the character archetypes of Alan Moore's
Watchmen and the terror of the constant threat of all out nuclear war during a heated arms race than discussing the dualistic nature of the Kool-Aid mascot's fight against his nemesis the Thirsties in
The Adventures of Kool-Aid Man and how his tireless, never ending struggle may be comparable to the plight of Sisyphus who was condemned by the gods to ceaselessly roll a boulder to the top of a mountain, only for it to fall again under its own weight.
vs.
A better example: If I'm going to write about the artistic merits of pop music videos, I'll go with Michael Jackson's
Smooth Criminal, not Rebecca Black's
Friday.
In fact, I think I'll watch Smooth Criminal right now.