When Billy Madison apologizes to Danny McGrath for being mean to him in high school, Danny crossed Billy's name off the People To Kill list. The eight other people left on the list are all crew members of the movie.
Bradley Whitford never actually saw the film after it was released. But according to his brother, people would often shout "Business Ethics!", a reference to the scene where his character is challenged to discuss that as a subject.
The 20 Categories listed during the final stage of the Academic Decathlon are: English Literature, U.S. Presidents, My Wife The Tramp, Astronomy, Baking, Burning Dog Poo & The Human Response, Reflections Of Society In Literature, Information Systems, Chemistry, The Roman Empire, My Spouse Is Sleeping Around, The French Revolution, Automotive Repair, 20th Century Poetry, Calculus, Needlepoint, Architecture, Business Ethics, Shakespeare, and I Married Common Street Trash.
The high school principal told Billy (Adam Sandler) and Eric (Bradley Whitford) at the start if the Academic Decathlon that if either one of them cheated they would be disqualified. Technically, during the chemistry event, Billy cheated.
Billy refers to the janitor as "Sideburns", based on his exaggerated sideburns. In The Wedding Singer (1998) Adam Sandler's character calls out a wedding attendee as "Sideburns" Lady based on her exaggerated sideburns.
At the Academic Decathlon, Billy Madison ends his uneducated Industrial Revulotion speech by yelling "Knibb High School Football rules!" which ends an awkward silence and the crowd erupts in cheers. The same thing happens in Bill and Ted's Excellent Adventure (1989) where a high school student (Ox Robbins) ends his uneducated speech with "San Dimas High School Football rules!" also ending an awkward silence and the crowd erupts in cheers.
The high school principal told Billy (Adam Sandler) and Eric (Bradley Whitford) at the start if the Academic Decathlon that if either one of them cheated he was just going to snap but did not say anything about disqualification.