Thanks for your feedback!
While it's harder to see, the grades bar does deplete, but not in a linear way. The game actually stores every score you get in the game and averages them together to get the final score. While this means that screwing up the final two levels won't tank your score, it also means that it's harder to see its impact after a few rounds.
Truth be told, the stats affecting gameplay actually never occurred to me; which is a disappointment because that's a fantastic idea! That's certainly something to include if I ever decided to expand the game.
Thanks again for your feedback!