The scene where Billy eats a banana and throws the peel in the trash is improvised, including Dean Parker's response: 'Pick up that banana'. Before the cameras started rolling, Sean Astin asked a prop guy for a banana and then proceeded to eat it during the take, even adding a roll of the eyes when asked to pick up the peel. Aside from Louis Gossett, Jr., actor reactions were pretty much genuine.
The character Derek, played by actor Shawn Phelan, goes by the nickname of Yogurt in the movie. This was a real life nickname given to him by his cast-mates. The story goes that all the young actors who played the main gang were in the back of a van, travelling on the way to the set to begin filming. As they were riding along, Phelan was eating a carton of yogurt, and the vehicle suddenly went over a big bump which caused the yogurt to spill out all over the actor, covering him in yogurt. The other guys found it so funny that they called him Yogurt for the rest of the shoot, which was then incorporated into the film.
The brick wall around the quad (which is actually known as "The Bull Ring") was constructed for the film, and was merely a brick façade on wood. At the end of filming, students celebrated by tearing it down.