I have to say, I think the tone of the mini-arcs is all wrong for this series. Charlie walking off into the distance to become a janitor just didn't have the big laugh that most episodes go out on. It's not the only time that's happened this year, either.
The show's priorities have changed. It's definitely evolved into character-based comedy, not gag-based, which means it's going to funny only to the portion of the audience that finds the characters themselves inherently funny.
I thought the scene with Charlie trying to eat the eraser was one of the funniest things the show has ever done. But that's definitely character-based - there's no actual "joke" per se in it, other than that Charlie is the type of guy who would do something like that (and the very deft comic timing of both Charlie Day and the guy playing the principal).