There are three Erics in Mrs. Jewls' class: Eric Fry, Eric Bacon, and Eric Ovens. Unfortunately no one realizes they're not all versions of the same person.
For example, everyone thinks all the Erics are fat. "There was a joke around Wayside School that if all the Erics were ever at the same end of the room at the same time, the whole school would tip over" (22.1). But Eric Bacon is the skinniest kid in class, though this doesn't stop his classmates from giving him the nickname "Fatso."
Eric Fry is actually fat, but he's also the best athlete in the class. But no one notices, because the other two Erics aren't good at sports, so Eric Fry is always picked last for teams and given the nickname "Butterfingers."
Eric Ovens is the nicest person in the class, but since the other two Erics are mean (due to being given totally inaccurate nicknames), everyone assumes Eric Ovens is also a jerk. The class, therefore, calls him "Crabapple."
All three Erics think it's an improvement to have nicknames, even though they're all totally wrong. At least this way they know when someone is talking to them.