Interesting continuity error

In an episode early in season 2, where Roger organizes the surprise 1 year anniversary party for Doug, we're apparently supposed to believe Doug has been there for 1 year. Also, in another episode that season, where Judy takes over the school's Founder's Day play, Skeeter makes reference to the season 1 episode where she did a show (when Doug introduced her) and refers to it as "last year."

Yet by the end of the Nickelodeon series, we basically see that the entirety of the 4 seasons comprise Doug's first year in Bluffington, since he was in the 6th grade throughout the show with the same homeroom teacher (Mrs. Wingo).

I guess they changed their minds on that midway through...maybe not yet wanting to make the logical changes they'd have to make if Doug were actually entering junior high in season 2.