- Emilio Estevez as Andrew Clark, an "athlete" who was sentenced to Saturday detention for taping another student's butt cheeks together.
- Paul Gleason as Richard Vernon, the grumpy domineering vice principal of Shermer High School who oversees the Saturday detention. He is combative towards the students, particularly John, and judges them based on their social stereotypes.
- Anthony Michael Hall as Brian Johnson, a "brain" who was sentenced to Saturday detention for bringing a flare gun that was to be used in a suicide attempt. His parents' constant pressure on him to get good grades drove him to this attempt.
- John Kapelos as Carl Reed, a janitor at Shermer High School who appears to be on friendly terms with Brian.
- Judd Nelson as John Bender, a "criminal" who was sentenced to Saturday detention for pulling the fire alarm and is the most argumentative towards Vernon. He has a physically and verbally abusive father who burned him on the arm with a cigar for spilling paint in the garage.
- Molly Ringwald as Claire Standish, a "princess" who was sentenced to Saturday detention for skipping school so that she could go shopping. Her parents, on the verge of divorce, endlessly use her to get back at each other during arguments.
- Ally Sheedy as Allison Reynolds, a "basket case" who was not sentenced to Saturday detention, and attends this one only because she could find nothing better to do. Her parents neglect her, which drives her to consider running away from home.
- Ron Dean as Mr. Clark, the father of Andrew who constantly pressures him to not "blow his ride".
- Mercedes Hall as Mrs. Johnson, the strict mother of Brian who pressures him to do well in his tests and appears unsympathetic to her son's suicide attempt.
- Mary Christian as Brian's sister.
- Tim Gamble as Mr. Standish, the condescending father of Claire.
- Perry Crawford & Fran Gargano as (respectively) Mr. and Mrs. Reynolds, the neglectful parents of Allison.
- John Hughes as Mr. Johnson (uncredited cameo), the father of Brian.
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