When it premiered on September 9, 1975, Welcome Back, Kotter was just another sitcom in ABC’s “new fall season” lineup. The cast was filled with television newcomers, and while the four main “students” had some film and Broadway chops, the star of the series had no acting experience whatsoever. Yet before the first season had ended, kids across America were parroting the mannerisms of Washington, Horshack, et al., the Sweathogs were being marketed in every conceivable medium, and John Travolta signed a $1 million three-picture deal with Robert Stigwood. Not bad for a show that never cracked the top 10 in the Nielsen ratings.

1. THE SERIES WAS INSPIRED BY GABE KAPLAN’S STAND-UP ROUTINE.

Kaplan was a star player on his high school baseball team and dreamed of someday playing in the major leagues. When he tanked at the San Francisco Giants’ spring training camp, he headed back east and took a job as a bellman at a resort hotel in Lakewood, New Jersey. After watching the touring comedians who performed there for a few months, he decided to take a stab at stand-up. He eventually developed a routine based on his experiences in a remedial class at Brooklyn’s New Utrecht High School and took his act on the road. Fellow Brooklynite Alan Sacks, who was working in Los Angeles as the producer of Chico and the Man, caught Kaplan’s performance at The Comedy Store at the urging of Freddie Prinze, and a TV sitcom pitch was born.

2. ANY RESEMBLANCE TO REAL PERSONS WAS STRICTLY INTENTIONAL.

Vinnie Barbarino (originally called “Eddie Barbarini” in the pilot script) was a combination of two real-life people: Kaplan’s fellow Sweathog Eddie Lecarri, and a tough kid named Joey Caluchi that Alan Sacks knew in junior high school. Freddie “Furdy” Peyton inspired Freddie “Boom Boom” Washington, and “Epstein the Animal” (as he was known at Kaplan’s alma mater) was transformed into the half Puerto Rican Juan Epstein at the suggestion of ABC’s then-head of programming, Michael Eisner. Only Arnold Horshack’s character retained his real-life counterpart’s name … although the original Arnold was so obnoxious that by the fourth grade, according to Kaplan, even the teachers began calling him “Arnold Horsesh**.”

3. ROBERT HEGYES ORIGINALLY AUDITIONED FOR BARBARINO.

In fact, he thought he’d landed the part until he arrived to shoot the pilot. He got into an elevator with Alan Sacks and John Travolta, and Sacks introduced Travolta to him by saying, “Epstein, this is Barbarino.” “No, no, no” Hegyes corrected him, “I’m Barbarino.” “No,” Sacks repeated, “you’re Epstein, this is Barbarino.” After a brief pause Hegyes asked Sacks, “Do I get the same pay he does?” When he was assured that their salaries were equal, he replied, “OK, it’s fine with me.”

4. THE SHOW WAS SUPPOSED TO BE CALLED SIMPLY KOTTER.

But when former Lovin’ Spoonful singer John Sebastian was commissioned to write the theme song, he found it difficult to find any appropriate words (Otter? Slaughter?) that rhymed with “Kotter.” Instead he composed a tune called “Welcome Back” that evinced a warm, nostalgic feeling of a man returning home to his roots rather than a classroom full of delinquents. The title of the series was duly changed, and “Welcome Back” went on to the top of the Billboard pop chart for one week in May 1976.

5. SOME OF CHARLIE’S ANGELS WANTED TO BE MRS. KOTTER.

Both Farrah Fawcett-Majors and Kate Jackson auditioned for the role of Julie Kotter. Marcia Strassman, the actress who landed the role, became good friends with Jackson, who gifted her with a motorcycle for her 30th birthday. Sadly, not long afterward, Strassman had a nasty accident while riding that left a gash on her cheek that required 10 stitches and eventual plastic surgery to repair.

6. HORSHACK’S LAUGH CAME FROM A SAD PLACE.

Ron Palillo was 10 years old when his father died of lung cancer. He developed a severe stutter as a result of the tragedy, and his mother sent him to acting classes hoping that it would help to correct his speech problem. When Palillo auditioned for the role, he made up the character on the spot and imitated his father’s wheezy, gasping voice as he struggled to breathe during the late stage of his illness to create Horshack’s trademark laugh.

7. THE ORIGINAL CATCHPHRASES WERE MUCH RUDER.

“Up your nose with a rubber hose” as a rejoinder was as ubiquitous as “Kiss my grits” and “Dy-no-mite!” in the late 1970s. The rhyming put-down came from Gabe Kaplan’s stand-up routine; it was called “ranking,” and the most famous rank at his school (usually uttered by Horsesh** when he was at a loss for words) was “Up your hole with a Mello Roll.” (A Mello Roll was an ice cream treat popular in Brooklyn and the Bronx.) The ABC brass decided that the Mello Roll rank, along with the others submitted by Kaplan, was inappropriate for primetime TV, so they softened them a bit. Not that “Off my case, toilet face” was a bouquet of roses.

8. THREE OF THE ACTORS HIT THE BILLBOARD CHARTS WHILE WORKING ON THE SERIES.

Marcia Strassman had tried her hand at a singing career back in 1967 with little success, and John Travolta would go on to have a few hits from the Grease soundtrack. But while Kotter was in its heyday, Travolta and two of his co-stars attempted to launch recording careers: Gabe Kaplan’s novelty single “Up Your Nose with a Rubber Hose” made it all the way to #91 in 1977, while Lawrence-Hilton Jacobs’ self-titled album nudged into the lower reaches of the Billboard Soul Chart in 1978. Predictably, John Travolta (who was already heading toward heartthrob status) had the most success, hitting #10 in 1976 with “Let Her In."

9. BOSTON-AREA RESIDENTS MISSED THE FIRST FOUR EPISODES.

When Welcome Back, Kotter premiered in September 1975, racial tensions were running high in Boston due to the court-ordered desegregation of public schools via forced busing. The head of Boston’s ABC affiliate decided that Kotter’s “cast of non-scholastic high schoolers might have an unhealthy influence on local students” and refused to carry the show. Four weeks later it became apparent that the Sweathogs had more in common with the Marx Brothers than with the Crips or Bloods, and the ban was lifted in time for the fifth episode to air.

10. GROUCHO MARX ALMOST MADE A CAMEO.

Gabe Kaplan managed to work his Groucho impersonation into almost every episode, and Robert Hegyes patterned Epstein after Chico Marx, so of course the two were excited when it was announced that Groucho had agreed to do a quick walk-on appearance. Marx was 86 years old at the time and in rapidly failing health. He made it to the studio, but he was barely able to walk (he leaned heavily on his “assistant” Erin Fleming) and seemingly unaware of his surroundings. The producers realized that he was in no shape to go on camera. Instead, he sat in Kaplan’s chair on the set and posed for a few pictures with the cast while Fleming pitched herself for a possible future Kotter appearance. Reportedly Marx’s appearance was so disturbing that the photos were never released.

11. THE FOUR SWEATHOGS WERE MORE POPULAR THAN FONZIE AT ONE TIME.

After the first episode of Welcome Back, Kotter aired, the four previously unknown stars were shocked to find that they couldn’t go out in public without being mobbed. The producers took advantage of their popularity and soon the Sweathogs’ faces were on everything from T-shirts to lunch boxes to board games. The network once had to fly the cast to LAX via helicopter so that they could catch their flight to New York on time. Looking down at the thousands of fans flocked around the airport, Robert Hegyes commented excitedly to Ron Palillo, “Ron! We’re the freakin’ Beatles!!” Palillo, who was more of a feet-firmly-on-the-ground kind of guy, replied, “Bobby, we’re not even the freakin’ Monkees.”

12. THERE WAS SOME SERIOUS DISSENT BEHIND THE SCENES.

The first rumblings of discontent came from Marcia Strassman, who realized by season two that her role as Mrs. Kotter was basically asking Gabe “And then what happened?” while he regaled her with one of his many stories about one of his many relatives. “I pray every day for a cancellation,” she told People magazine in 1978.

Kaplan was in a power struggle with producer James Komack, who fired most of the writing staff who’d been with the show from the start at the beginning of season four and hired the writing team from The Carol Burnett Show. Kaplan objected to the shift from high school student/teacher issues to more slapstick/blackout sketch comedy and he appeared in only a handful of episodes during the final season as a result.