In ‘Blockers,’ John Cena, Leslie Mann play parents out to stop a prom-night sex pact

I went into “Blockers” cringing and came out smiling, which says more about me than it says about the movie. But that’s how moviegoing works. We’re pre- judgy that way. And “Blockers,” the feature directorial debut of “Pitch Perfect” screenwriter Kay Cannon, turns out to be well-aware of that double standard, in a consistently funny and rather sweet fashion.

This is what Hollywood used to call a generation gap comedy, although imagining David Niven in “The Impossible Years” ( 1968) entering an alcohol- enema contest at a kegger is, well, not imaginable, and only serves to remind us that times, mores and the lines in the sand of tastelessness are always shifting.

The movie is set in Waukegan, Ill., and environs. ( It was shot in cost- effective Georgia; the generically lush, peach-tree aura does not evoke Waukegan, for the record.) Leslie Mann, Ike Barinholtz and John Cena play the blockers of the title, friends since their daughters, played by the extremely deft trio of Kathryn Newton, Ger- aldine Viswanathan and Gideon Adlon, bonded in elementary school.

Now they’re graduating seniors. One day at lunch the girls make a pact to lose their virginities on prom night.

“I’m having sex,” Newt on’s character, Julie, declares. Great, says Sam, played by Adlon. “I’m having soup.”

With that one exchange the movie had me: The teenage girls act, and interact, like human beings on planet Earth, fully free of the nagging John Hughesian archetypes of “babe” or “wallflower” or “uptight scold.”

Some casual digital monitoring reveals the plan to their parents. Overprotective Lisa ( Mann), still smarting from her own dubious choices at her daughter’s age, panics nearly as much as Mitchell (Cena), despite the protestations of Mitchell’s wife (Sarayu Blue) that he’s acting like an idiot, treating their daughter like a helpless flower to be protected. The party-boy slob Hunter ( Barinholtz) feels differently; he wasn’t much of a family man when he was married, but he’s striving for a connection with his daughter, and he knows she’s about to come out of the closet. For a while, he’s the voice of reasonable opposition to the other parents’ plans to block those kicks. (Screenwriters and brothers Brian and Jim Kehoe can’t really figure out how to sustain that opposition, so they simply drop it, midmovie.)

From there, “Blockers” becomes a tracking device of a sex comedy. The dithering middle- age killjoys follow the girls and their dates from prom to lakeside after- party to hotel bash. Some of the gags are centuries old (Mann hiding under the bed where her daughter’s about to Do It); other scenes redi- rect the movie to a fresher, more incisive place.

In no way, shape or form is “Blockers” a female “Porky’s,” or a female version of any traditional boys club scenario. It’s closer in spirit to something like “Superbad” in its mixture of commercial hooks and character-driven detours. Director Cannon’s technique is jumpy and rudimentary at this stage, but she doesn’t push the young women or men into farce behavior, even when her direction is visu- ally obvious.

A little of Barinholtz goes a pretty good distance for me. But sharing scenes with Mann (who has the timing of a wizard) and blocklike Cena (funny just standing there, with his “cop haircut” and perpetually aghast reactions), he’s what the movie needs: a relaxed wild card. The girls’ friendship, however, is what makes “Blockers” more than an elevator pitch. One trio gets ‘em in the door; the other trio keeps ‘ em there.

UNIVERSAL PICTURES From left, John Cena, Geraldine Viswanathan and Miles Robbins in “Blockers.”