So how long has the Dean been at Greendale?

Or how bad is Pierce's memory exactly, cause I may have found an inconsistency in the story.

In the second 6th episode Pierce doesn't know who the dean is, but then in the season 2 finale Pierce reveals he's been going to Greendale for 12 years. So that made me think season 1 would maybe be the first year for the Dean somehow, but then in episode 217 at the election the Dean says:

"This is why we abolished student government in the first place, this is exactly what happened 10 years ago."

But that doesn't have to mean he was there for it, so tell me what you all think.