Audio problem: When the waitresses are performing their song, you can tell they are lip syncing to a recording even though they are supposed to be giving a live performance. The vocals have a noticeable amount of reverb and are mixed evenly with the instrumental track despite the fact they aren't even using microphones. (00:06:49 - 00:09:00)
Character mistake: When Sarah the waitress is filling up Cory and Shawn's coffee cups, she puts hardly any coffee in them, with Cory's barely being a couple of drips. A waitress that works on tips wouldn't be so lackadaisical about filling a customer's coffee cup. Cory and Shawn just sat down as well, so it's not like she was just topping them off. (00:02:58)
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.