Continuity mistake: Shawn steals some fruit just before the interview starts. He empties the whole bowl into his backpack. When the interview starts, the bowl is full again, with the exact same fruits that were just taken by Shawn.
Character mistake: After the production assistant walks away from the catering table, he walks into the area where the Matthews are being interviewed. Any production assistant would know better than to just casually walk right into the shot when a live televised interview is going on.
Continuity mistake: When Shawn is loading up his backpack with food from the catering table, a production assistant emerges from the kitchen and asks him if he's part of the interview. After Shawn says he is, the assistant walks away towards the front door. When the shot changes to the couch where the family is being interviewed, the assistant is nowhere to be seen, even though he walked right into the area of the shot. When the shot changes yet again, the assistant is back by the catering table, standing with his arms folded.
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.