It would work, but it won't last as long as Arthur did as a third grader. A perfect example is what happened to the Rugrats.
See that's another thing: it wouldn't have to. The spinoff would have a fixed and strict amount of seasons, much like Scooby-Doo: Mystery Incorporated. Something like: first season = sophomore year, second season = junior year, and the last season = senior year, with a freshman flashback episode somewhere in there.
I dunno, I don't even see it being able to last that long. In my mind, if they were to do a spinoff show, I doubt it would have the umph to last more than two seasons before people were done with it...that is, if they would even give it a chance to start out with.