Nicolas Cage was originally considered for the role of John Bender but the production couldn't afford his salary at the time. John Cusack was originally cast as John Bender, but John Hughes decided to replace him with Judd Nelson before shooting began.
In 2010, Molly Ringwald and Anthony Michael Hall told Vanity Fair magazine that John Hughes was receptive to actors' improvisations, and some of them (including Brian's reason for having a fake ID - "so I can vote") made it into the final film.
When Principal Vernon is caught looking through confidential files by Carl the janitor, the position of Carl's arm as the two discuss the situation changes from on top of the cabinet to inside the drawer with each cut.
John (Bender) pulled the fire alarm causing the whole school to be evacuated. Due to his language and attitude towards Principal Vernon he is given additional detentions for nine weeks afterwards. Claire skipped class to go shopping. Presumably, she thought she would never end up in detention because she believed her father would have the power to see that she wouldn't have to spend time somewhere she felt she did not belong. Andy taped medical tape to a weaker students butt cheeks and ripped it off, causing the student's skin to rip off also. Andy did this to impress his father who expected him to act the same way he had in High School. Brian brought a flare gun to enact his plans to commit suicide. He was planning to do this because he failed Shop Class and knew the hassle he received from his parents for failing one class would be more than he could deal with emotionally. Allison's reason for being in detention is revealed later than the other characters. Finally, she claims she's there because she had "nothing better to do"
Q
What does Bender's joke mean?
A
Bender's joke begins ""A naked blonde walks into a bar with a poodle under one arm and a two-foot salami under the other. She lays the poodle on the table. Bartender says,'I suppose you won't be needing a drink.' Naked lady says..." He then falls through the air vent and in due to his shock, forgets about telling the joke yelling "Oh shit!" The next lines uttered by him are "Forgot my pencil" by means of an explanation to his confused classmates as to how he got back into the library. The joke was ad-libbed by Judd Nelson and as thus, does not have an actual punchline.