Continuity mistake: Across the show, the number of kids in a class/what kids are in which class varies. For example, in "The Great Can Drive", the Ashleys are in Ms. Furley's class. However, in almost every other episode besides this one, the Ashleys are all in Ms. Grotke's class, and the exception are a couple of episodes in which 2 Ashleys are in Ms. Grotke's class, and the other 2 Ashleys are in another class (which is not specified by name), making a serious consistency error.
Continuity mistake: In the beginning, right as Mrs. Grotke asks for Gus to describe his "personal journey", there is a short kid with a white t-shirt and dirty blonde hair, with the globe behind his head. As the kid asks if they can go to recess now, the globe has moved way to the left.
Continuity mistake: When TJ makes the morning announcements, in the first scene of the classroom we see a boy in a blue hat sitting in the back row. After TJ says that the kids can refuse to answer a teacher, the same kid is suddenly sitting in the second row so he can slap hands with someone in the first row.
Continuity mistake: When TJ's friends climb on on the jungle gym to try and take TJ's place, we see that Gretchen is the last one up and there is no one on the bars behind her. But in the next long shot, Mikey is right behind her. Then when it shows a close up of them, Mikey is gone again.