The state of movie comedy is so frail that the amiable, exuberant, conventional new comedy “Blockers,” which maintains good spirits and sustains good gags without excessive contrivance, comes off as a welcome, robust pillar of the genre. The film, directed by Kay Cannon (it’s her first feature), has a premise of classic simplicity that relies on the themes of old-school farce and the narrowly conventional moralism that traditional comedy lampoons. Three comfortably middle-class teen-age girls in an Illinois suburb—Julie (Kathryn Newton), Kayla (Geraldine Viswanathan), and Sam (Gideon Adlon)—have been friends since their first day of school. Now they’re high-school seniors, still the closest of friends—and the day of their senior prom, they make a pact to lose their virginity that night.

The girls’ parents bonded, too—especially Julie’s mother, Lisa (Leslie Mann); Kayla’s father, Mitchell (John Cena); and Sam’s father, Hunter (Ike Barinholtz). Now, the day of the prom, Lisa happens to hear pinging noises from Julie’s computer—she has left it open, and it’s loading a series of text messages in which Julie, Kayla, and Sam plot the night’s sexual exploits. The panic-stricken Lisa gets in touch with Mitchell and Hunter so that they can team up to put a stop to their daughters’ plans (or, as Lisa says, “cock-block those motherfuckers”).

The three girls’ own plans are clear. Julie has a longtime boyfriend, Austin (Graham Phillips). Kayla, the sharpest wit of the three, has no boyfriend and picks out her target seemingly at random from the cafeteria: Connor (Miles Robbins), the free-spirited stoner known as the Chef, for his habit of cooking a wide variety of drugs into a wide variety of foodstuffs. As for the quiet, moderately introverted Sam, she’s a lesbian who hasn’t come out yet, not to her parents and not to her friends; though she picks out a boy, the nerdy Chad (Jimmy Bellinger), for her prom date, she’s attracted to another senior-class girl, Angelica (Ramona Young).

“Blockers,” written by Brian Kehoe and Jim Kehoe, is a riff on post-Apatovian ribaldry that shares Apatow’s sense of traditional wisdom—that, like Apatow’s films, invests its sexual extravagance with ethical substance and explores the emotional truth at the heart of erotic experience. From the start, sexuality uneasily intrudes on the relationship of parents (especially fathers) and daughters in a comedic flash when Mitchell discovers what he takes to be Kayla’s vibrator (it’s actually an electric-toothbrush handle)—which is to say that, from the start, the parents’ fretful and panicky quest to inhibit their daughters’ sexual experience is self-evidently wrongheaded, moralistic but immoral.

What’s fascinating and peculiar about “Blockers,” though, is that, unlike in Apatow’s comedies, the kids who are planning to have sex aren’t the sources or the subjects of the movie’s ample and energetic humor. The three young actors are lively, but only one of the characters—Kayla—gives any of them much comedic material to work with. All three of the girls are socially selected—they’re all good students, hardworking, dutiful; none is any trouble to her parents; all have their feet on the ground and their gazes fixed on future accomplishments. There’s no drama in Julie’s relationship with Austin; Sam’s drama is conspicuous, but it’s hardly a spoiler to say that it gets resolved with warm feelings and no trouble. Kayla, who is both sexually curious and romantically uninvolved, gets the sharply flamboyant lines and the mildly risqué situations, and Viswanathan, with a flair for casually delivered hyperbole, makes the most of them.

By contrast, it’s the three parents at the center of the action who are both the spark and the target of the movie’s comedic inspirations—and what’s most notable about those inspirations is that they involve a series of gleefully elaborate sight gags and rowdy physical humor, with pratfalls and shocks, tumbles and calamities that are choreographed with an unusual precision and compositional flair.

Barinholtz’s Hunter is a goofball, the eternal youth who brashly but goofily tries to fit in among the teen-agers, and whose conduct—he’s said to be divorced because he had an affair with one of Sam’s babysitters—reflects the perpetual adolescence that he flaunts. Mann has long been among the most scintillating of comedic actors (even in a noncomedy, such as “The Bling Ring”). Here, she invests the role of Lisa with a comedic willfulness that’s built from the character and her backstory—the tale of how Lisa, an ambitious young college student, became pregnant, lived as a single mother, and now hopes to spare Julie a similar fate. As for Mitchell, played by Cena (who, of course, is a former pro wrestler), his character, too, has comedic divisions built into it—he’s a hefty, hulking, highly disciplined athlete who has given Kayla his love of sports as well, but he’s also a dewy-eyed sentimentalist who cries easily. Mitchell, with his bluff but sweet manner, turns out to be the butt of the movie’s wildest physical humor.

There are other parents on hand as well—Marcie (Sarayu Blue), who is Mitchell’s wife and Kayla’s mother; Brenda (June Diane Raphael), who is Sam’s mother; and Frank (Hannibal Buress), Brenda’s husband and Sam’s stepfather—but they figure only slightly in the action and even less in the comedy. Rather, another pair of adults—a couple of randy neighbors (Gina Gershon and Gary Cole) whose sex play is meant to keep the spark alive in their marriage and whose house holds secrets that Lisa, Mitchell, and Hunter are pursuing—prove more crucial players in the movie’s comedic set pieces.

For all its imaginative contrivances and mildly smutty comedy, though, “Blockers” is a movie that remains amazingly, dishearteningly pure. The cast of the film is multicultural, but the story is utterly noncultural, set in a generically prosperous suburb in which neither parents nor teen-agers have very much inner identity at all. The high-school students break down very quickly into types, but those perennial stereotypes offer no substance, no difference, no divergence—no individualism and no solitude. “Blockers” celebrates the smartness of smart kids, the overwhelming practicality of teens who are part of the popular set and are, from the start, candidates for the yearbook’s most likely to succeed. These so-called “motherfuckers,” and the movie’s sentimental attachment to their self-aware wisdom, arouses nostalgia and yearning for the wilder adventures of actual fuckups.