There actually was a scene of him with the student body and he wasn't dressed like a teacher or faculty member, giving no visible proof that he was with the faculty or that he wasn't, as opposed to Cheerilee who was visibly one of the faculty members int he library. Until I can get more proof, though, I'll let it go here.
I wouldn't pay attention to what the movie says about their ages. I mean, the CMC are in Highschool. Highschool! That pretty much throws the whole age thing out the window. (Also awesome picture. Love Scootaloo's expression)