Other mistake: At the start when the gang change the time on the clock in the classroom, it wouldn't control the main school clock, plus in every other episode the clock has been operated by The Box on the side of the school, as shown in " The spy who came into the playground".
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.
Plot hole: Whenever Gretchin calls Mikey up to the stand to explain what he saw, he says that he had followed Spinelli and Randall to the wooden fence, where he Saw Randall on his knees in front of Spinelli, begging for forgiveness, then Mikey began running to the other side to stop Spinelli from pummeling Randall with the "rock" but didn't make it in time. We then later find out that Spinelli had actually rescued Ms. Finster's cat out of a tree and Randall hit himself with the rock out of spite. But it is impossible that Spinelli could have given up on hurting Randall, climbed up a tree, rescued a cat, climbed down, handed the cat back to Ms. Finster, gotten praise from Ms. Finster, then have Randall attempt to pick up a big rock, then get a smaller one and hit himself with it in under 3 seconds (the duration of how long Mikey ran before he heard Randall's scream).
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.
Factual error: At the end when Ms. Finster tells Randall to give her the names of the kids involved with the ball incident, she says to list them in alphabetical order. The first name Randall says is "Dettweiler, T.J." But "Blumberg, Mikey" would actually come before that, and given what we know of Randall, this isn't the kind of mistake he is likely to make.