Continuity mistake: When Cory is asking Mr. Feeny to call his parents to pick up him and Shawn, Feeny can be seen removing the white drape from the sofa. When the shot changes, Feeny has instantly removed the drape from the chair across from the sofa as well.
Continuity mistake: When Cory and Shawn mistakenly believe Mr. Feeny opening the door to his cabin to be an axe-murderer, Cory throws his gloves at a nearby chair to grab something to defend himself with. One glove hits the floor in front of the chair, while the other lands on the chair and slowly falls off. A couple of shots later, the glove is back on the chair, while later shots show the other glove to be on the floor by the chair's right armrest.
Continuity mistake: During the first sequence of Eric and Frankie's game of pool, after Frankie's shot, the cue ball is shown slowing to a stop near the center pocket of the table opposite from where Eric is standing. A few shots later when Eric is taking his shot, the cue ball is suddenly near him.
Chosen answer: In the present day, with the United States abiding by more stricter school health laws, for the most part this wouldn't be in school cafeterias as many cannot even have carbonated beverage machines now. However, in the 90s when the show takes place, it wasn't unheard of for there to be coffee machines in the cafeteria of small schools where the teachers eat with the students as we see them do often in the show. The idea being that the cafeteria was small enough so a teacher or hall monitor could catch a student before they could drink the coffee.