In one quiet scene in “Strangers With Candy,” Amy Sedaris, playing Jerri Blank, an ex-prostitute and ex-drug user who has returned from a 32 year binge to start her life over as a high-school freshman, repeatedly tries – and fails – to throw popcorn into her own mouth. It is a testament to Ms.Sedaris’s perfect comic control of her body, still graceful under a lumpy fat-suit, that even this simple sequence of wide-eyed, optimistic, and yet hopeless flailing is hilarious. Ms. Sedaris co-wrote the movie, along with her fellow Second City alumni, Stephen Colbert and Paul Dinello (who also directs).The collaborators have clear ideas of each others’ strengths, from Ms. Sedaris’s physical skills to the convoluted intellectual humor of Mr. Colbert’s dialogue – here showing the same talent evidenced in Comedy Central’s “The Colbert Report.”

The three have long worked together, and co-created the television series “Strangers With Candy” in 1999. The show enjoyed a cult following during its three seasons on Comedy Central, and was inspired by a 1970s education video about a real-life ex-prostitute, drug user, and con who toured high schools as an inspirational speaker. The collaborators decided to see what would happen if a similar character – “a boozer, a loser, and a user” – decided to go back to school for good, not just for a visit.The result, both for the show and now the movie, is an extended parody of the after-school special.

The comedy focuses not on the students but rather on the selfishness and arrogance of the adults who manipulate them, and each other, usually with amusing disregard for logic, as when Mr. Colbert’s character, the secretly gay biology teacher, says to Mr. Dinello’s archetypal art teacher, with whom he has been having an affair: “I need more out of this relationship than I’m willing to put in.”

The movie is a prequel to the series, and covers Jerri’s first days back in school and her ill-considered attempts to fit in and be popular. Her prison days have equipped her to deal with school bullies (she merrily bangs her locker shut on the head of one of them) but the collision of her past behaviors with present circumstances are usually more awkward and lead to most of the big laughs in the movie. Especially funny, and also extremely vulgar, is her relationship with Tammi (the wonderful Maria Thayer), a geeky freshman girl who Jerri treats as both best friend and fresh meat.

The plot ostensibly revolves around the competition leading up to a science fair – Jerri wants to win to prove her redemption, but the principal has brought in a ringer, Matthew Broderick, to coach a stealth team and increase the school’s funding.

But the story line mostly serves to provide a loose structure for somewhat disconnected episodes, which contain the real comic gems of the movie: catty hysterics in a teachers’ lounge complete with a piano player and full bar, Jerri’s frantic attempts to seduce a hunky football player (Chris Pratt) who only wants her science project secrets, the antics of a driver’s education teacher (Justin Theroux) who crash-lands himself on the hoods of unsuspecting students’ driving simulators. Sarah Jessica Parker joins the cast as a school counselor who demands that her clients fork over their lunch money as “tips,” Allison Janney and Philip Seymour Hoffman are love-addled school-board members, and Kristen Johnston is a wheelchair-bound physical education coach who proves that there’s nothing like releasing a few wild bulls in the gym to improve students’ track times.

Especially with the addition of these celebrity guest stars in well played cameos, “Strangers With Candy” is sure to satisfy both fans of the series and newcomers. And this despite the filmmakers having disregarded the philosophy of their main character, who is known for saying: “If you’re gonna reach for a star, reach for the lowest one you can.”