In this episode, when Mikey is seen as a kindergardener, he looks nothing like Mikey, or like he appeared when he remembered his experience in the Kindergarden Derby.
TJ and the gang can't rememeber anything about their year in kindergartern, ie TJ doesn't remember how he used to be a wimp, but Mikey could remember the Kindergartern Derby in the episode of the same name, and the 5 could remember how they first met back in kindergartern in The Great Jungle Gym Stand Off.
This appears to take place while T.J and the gang are in the 4th grade.
We find out that the 'J' in T.J. Detweiler stands for Jasper.
The title refers to Paul and Joe's other show All Growed Up.
If you wonder why Gus is on the cover when he supposedly entered Third Street at 4th grade. Well in the DVD, Gus went to Third Street during Kindergarten but only for a couple of weeks and only he remembers kindergarten while the rest of the gang doesn't remember a thing about that year which explains why Gus is on the cover.
This film has flashbacks to 3 episodes, The Legend of the Big Kid, Wild Child and Kindergarten Derby, and a new episode, when T.J. was in kindergarten with his freinds.