St. Joseph's Preparatory School is an all boys prep school in Philadelphia attended by most of the members of the Gang, along with a number of other recurring characters on the show. Dee and other female characters attended Notre Dame High School.
- Dennis Reynolds - Had always considered himself a pretty popular guy back in high school. ("Underage Drinking: A National Concern") But it turns out that he was merely an ordinary loser, who called himself a "Golden God" and referred to the other students as his "minions", but then he'd just go and hang out with other school losers. ("The High School Reunion, Part 2: The Gang's Revenge")
- Deandra Reynolds / Dee (aka "The Aluminum Monster") - Wore an aluminum back brace because of her scoliosis. According to her, she went out "with tons of guys" - in reality, everyone was just afraid of her. She also didn't go to the prom - because her Mom called her fat and resulted in her crying all night long. ("Underage Drinking: A National Concern")
- Charlie Kelly (aka "Dirtgrub") - According to Dennis, Charlie was popular in the way that a clown is popular and that all the guys knew Charlie wasn't a threat as he couldn't score with their girlfriends. Just like Dee, he wasn't present at prom. ("Underage Drinking: A National Concern") Jimmy Doyle said Charlie had a lot of acne back then. ("Charlie Wants an Abortion") Popular kids (like Adriano) were able to make him do anything - for example, eating a spider. ("The High School Reunion")
- Ronald McDonald / Mac (aka "Ronny the Rat") - Dealt weed to everyone; he thought everyone liked him but everyone really thought he was an asshole. ("Underage Drinking: A National Concern") He became a main drug dealer of the school when he told on the other main drug dealer. ("The High School Reunion")
- The Waitress - Sat next to Dee in Trigonometry. ("The Waitress is Getting Married") Most forgettable girl in the school. ("The High School Reunion")
- Matthew Mara (aka "Rickety Cricket") - Wore giant leg braces and was (and still is) in love with Dee. They were in physical therapy together twice a week after school. She promised to kiss him if he ate horse dung. He did so, but she refused to kiss him because his breath smelled like shit. Dennis and Mac used to teabag Cricket extensively in high school (and they still do at every chance they get). ("The Gang Exploits A Miracle")
- Ingrid Nelson (aka "Fatty Magoo") - Was fat (because of her thyroid problem) and Dee's best friend. ("The Aluminum Monster vs. Fatty Magoo")
- Stacy Corvelli - Slept with Charlie. Tricked him into thinking he was the father of Tommy - her hellion son. ("Charlie Wants an Abortion") According to Dennis, she "was a slut". In a deleted scene on the Season 5 DVD it is implied that Stacy is the only woman Charlie has ever had sex with and that he did not enjoy it.
- Jimmy Doyle - Stacy's loser ex-husband who abandoned her and their son Tommy. ("Charlie Wants an Abortion")
- Brad Fisher - Everybody used to make fun of him: he had a lot of acne. Both Dee and the Waitress dated him, then broke up with him because of his "pizza face". ("The Waitress is Getting Married")
- Nikki Potnick - Girl, who's improvidently lent her car to the Gang. ("Underage Drinking: A National Concern")
- Tim Murphy - Guy, who didn't sleep with Dennis' prom date. ("The High School Reunion, Part 2: The Gang's Revenge")
- Mark Dunlap - Young cop who knew Dennis and remembered the Aluminum Monster and Fatty Magoo. ("The Aluminum Monster vs. Fatty Magoo")
- Maureen Ponderosa - Dennis' high school sweetheart. Had a dead tooth. ("Mac Fights Gay Marriage")
- Bill Ponderosa - Maureen's brother. According to Dee, he was "hot with great buns". ("Mac Fights Gay Marriage")
- Peter Schmidt / Schmitty - The original fourth member of the Gang. He used to be roommates with Charlie. The Gang got tired of Schmitty and resulted in Dennis kicking him out of a moving vehicle and telling him "You're out of the Gang!".
- Adriano Calvanese - Was a really "cool kid". He's coined a Dee's nickname "Aluminum Monster". ("The High School Reunion")
- Dooley - Mac & Charlie's friend who's killed himself. ("Dennis and Dee's Mom Is Dead") Replacement member of the "Freight train". ("The High School Reunion, Part 2: The Gang's Revenge")
- Psycho Pete - Leader (or the "psycho back") of the "Freight train"-gang, which also contained Mac, Charlie and Dooley. After school, Psycho Pete cut his family into a little pieces and ate them for Christmas dinner. ("The High School Reunion, Part 2: The Gang's Revenge")
- Dr. Larry Meyers - Dee's high school drama teacher who inspired her love of acting but in fact, was just an enthusiastic alcoholic. ("The Gang Gets a New Member")
- Ms. Klinsky - The school librarian who 'bedded' Dennis when he was 15 and apparently looked like Rick Moranis. ("Dee Made a Smut Film")
- Liam McPoyle and Ryan McPoyle - Were in Coach Murray's elementary school gym class with Mac and Charlie Kelly. ("Charlie Got Molested")
- Artemis Dubois - she's familiar with Matthew Mara, and she's aware about the Gang's high school lore such as why Waitress and Dee broke up with Brad Fisher. On the other hand, Dennis and Waitress at first seemed to be unfamiliar with her; ("Charlie Has Cancer", "Who Pooped the Bed?") most glaringly, she absent at the high school reunion.
- Ricky Falcone - Mac's family robbed his house on Christmas of 1985. Little Mac got himself an "Omnibot"-toy there - later, he's always thanked Ricky at school for it (most likely, it was elementary school). ("A Very Sunny Christmas")
- Jamie Nelson - Mac & Charlie's friend who loved skateboarding and drowned in a diving pool. ("Mac and Charlie: White Trash")
- Davy Mara - Matthew Mara's older brother, who is aware of his younger brother's nickname of Rickety Cricket, as well as being familiar with the Gang.
- Saint Joseph's Preparatory School is an actual school in Philadelphia. Rob McElhenney is an alumnus, class of 1995.
- It has been implied that Dennis and Dee didn't meet Mac and Charlie until high school.
- Mac and Charlie met in St. Vincent's elementary school. ("Charlie Got Molested")
- Dennis, Lil' Kev and presumably Dee went to Waldron Elementary school. ("Sweet Dee's Dating a Retarded Person")
- Mac mentioned to Dee that he and Charlie went to a Catholic school. ("Charlie Wants an Abortion")
- Charlie slept with Stacy Corvelli back in high school. Dennis told Stacy that he remembered her and Stacy asked Dennis if he went to "St. Joe's". ("Charlie Wants an Abortion")