“I got this idea from the romantic comedy ‘American Beauty.’ ” – Teenage girl, spreading rose petals on a bed.
“Uh, did you watch that movie all the way through?” – Her boyfriend.

Every once in a while, the raunchy and putatively edgy R-rated comedy “Blockers” delivers a funny moment like that. I laughed out loud six or seven times during the film — probably four times in the first 20 minutes, and then, not so much.

Despite the best efforts of reliable comedic veterans Leslie Mann and Ike Barinholtz, not to mention a game and always likable John Cena and terrific talents such as Hannibal Buress, Gary Cole and Gina Gershon in extended cameo roles, “Blockers” becomes less interesting and less funny as the onscreen hijinks grow more outlandish and stupid and demeaning and crotch-oriented.

Here’s another problem. This is a teen comedy, but all of the aforementioned actors are the adults playing the parents. The young actors playing the three best friends and their love interests are just … OK. None of the performances stands out. Even the kids playing the obligatory comic-relief characters — the dork with light-up shoes who wants to perform a big solo dance at prom, the short and slightly round guy who insists on wearing a too-small fedora and is always cracking wise — are adequate but not memorable.

Most of “Blockers” takes place during the night of the senior prom. Best-friends-since-first-grade Julie (Kathryn Newton), Kayla (Geraldine Viswanathan) and Sam (Gideon Adlon) make a pact: They’re all going to lose their virginity tonight. They’ll celebrate the anniversary every year at Olive Garden! Breadsticks forever!

High fives all around.

Julie is the only one with a longtime boyfriend: a very nice boy named Austin (Graham Phillips). I really can’t tell you much more about Austin, because he’s rarely onscreen. Even when he’s onscreen, he barely registers. (Sorry Austin, but 20 years from now, Julie is going to be hard-pressed to remember your name.)

Kayla has decided she’ll sleep with Connor (Miles Robbins), a borderline creepy stoner who sports a man-bun and is known as “The Chef” because he bakes serious drugs into all of his creations.

“He almost killed me!” one dope says of Connor at a keg party. Gee, that’s hilarious.

And then there’s Sam, who is gay but hasn’t come out yet. Sam has a huge crush on a girl (Ramona Young), but she’s going to at least try to have sex with her date (Jimmy Bellinger) on prom night to make sure she’s not interested in boys.

When Julie’s mother Lisa (Leslie Mann), Kayla’s father Mitchell (John Cena) and Sam’s dad Hunter (Ike Barinholtz) learn of the pact (by reading Julie’s text messages on her laptop), they spring into action, determined to save their daughters, who are about to graduate high school and are perfectly capable of making their own choices and aren’t necessarily in need of saving. It’s not as if the guys are predators or are tricking the girls. It’s THE GIRLS who have made this pact.

The parents are the ones who are unhinged, stalking their kids at the prom, then at a party at a lake house and then at an after-party in a downtown Chicago hotel. (Like “Office Christmas Party” and the “Bad Moms” movies, “Blockers” is yet another Chicago-set comedy filmed primarily in Atlanta.)

Lisa sneaks into the hotel room where her daughter is about to lose her virginity. Mitchell busts down door after door in the hotel, searching for his daughter. (Somebody call security!) Hunter, who has been an absentee father for nearly the entirety of his daughter’s life, tracks her down and has a bonding moment with her that rings absolutely false.

There’s a chugging contest where the beer is not consumed orally, and we’ll leave it at that. There’s a projectile vomiting sequence, and I would have been surprised if there hadn’t been a projectile vomiting sequence. There are not one but two weird detours to the home of Austin’s parents (Gary Cole and Gina Gershon), free spirits who share everything with their son. The second visit includes a close-up of testicles being squeezed extra hard — by a blindfolded woman who thinks she’s touching her husband but is in fact fondling a stranger.

To its credit, “Blockers” calls out the parents on their double standard. Would they be so horrified about the idea of their children losing their virginity on prom night if they were boys?

And all three of the girls are smart, confident, funny — and perfectly comfortable using the kind of language we normally hear only from the guys in R-rated teen comedies. This despite the fact each has a parent who engages in embarrassing, shameful, horrific, memory-scarring and sometimes criminal activities on prom night because they don’t trust their own children.

Even in a broad comedy, there’s something sad at the core of that.

★★

Universal Pictures presents a film directed by Kay Cannon and written by Brian Kehoe, Jim Kehoe, Jon Hurwitz, Hayden Schlossberg and Eben Russell. Rated R (for crude and sexual content, and language throughout, drug content, teen partying and some graphic nudity). Running time: 102 minutes. Opens Friday at local theaters.