Carl the Janitor (John Kapelos) only has two scenes in the movie. But they're still pretty memorable. He seems to represent an adult figure who isn't as repulsive as Vernon. Nevertheless, Bender acts like a jerk toward him:
BENDER: No I just wanna know how one becomes a janitor because Andrew here is very interested in pursuing a career in the custodial arts...
Bender's basically mocking Carl for being a janitor and predicting the same fate for Andrew. This is kind of ironic, given that Bender doesn't exactly have more social cache than a janitor. Carl responds with a magisterial rejoinder:
CARL: Oh, really? You guys think I'm just some untouchable peasant? Peon? Huh? Maybe so, but following a broom around after s***heads like you for the past eight years I've learned a couple of things. I look through your letters, I look through your lockers. I listen to your conversations. You don't know that, but I do. I am the eyes and ears of this institution my friends. By the way, that clock's twenty minutes fast!
So that's Carl's first scene. Later on, in his second and last appearance, he first tries to blackmail Vernon into giving him money when he catches Vernon looking at some high school kid's confidential mental health files. Then, they have a little chat.
Carl, being lower down the social hierarchy, has more sympathy with the students—he seems to get what's going on with them. But Vernon views them as a plague, a force that threatens his wellbeing deeply. After Carl says he wanted to be John Lennon when he was a kid, Vernon says:
VERNON: Carl don't be a goof! I'm trying to make a serious point here. I've been teaching, for twenty-two years, and each year these kids get more and more arrogant.
CARL: Aw bulls***, man. Come on Vern, the kids haven't changed, you have! You took a teaching position, 'cause you thought it'd be fun, right? Thought you could have summer vacations off and then you found out it was actually work, and that really bummed you out.
Carl is calling Vernon out on his lack of empathy with the kids. Vernon's just in it because he wanted a job with some nice benefits. But Carl remembers what it's like to be in high school because, in a sense, he never left. Briefly, at the beginning of the movie, we see an image of Carl in a class yearbook composite picture on the wall. So, he's remained at his alma mater, albeit in a different capacity.