“Fuck ‘Glee,'” Channing Tatum’s hulking undercover cop mutters early on in “21 Jump Street,” having disguised himself as a teenager for a high-school drugs bust, only to discover that his letterman-jacketed jockishness no longer carries the social cachet it did in his youth. It’s a throwaway line that nonetheless unlocks several suspended levels of socio-cultural awareness in Phil Lord and Chris Miller’s enthusiastically goofy spin on the long-buried youth TV series of the same name — a blithe barrage of wildly variable gags that would no more admit to such awareness than have Tatum and his partner in bromance, Jonah Hill, kiss on screen.

“Glee” would, of course, and therein lies the point. Perhaps the most conceptually playful and self-sustaining entry in the recent mini-genre of 1980s TV reboots, “21 Jump Street” employs its dodgy cultural lineage less as nostalgia kick than as conversation starter: amusing (and genuinely flummoxing) Johnny Depp cameo aside, the film is only incidentally interested in its source material, and far more preoccupied with the tension between between past and present adolescent generations.

Tatum’s offhand “Glee” diss may amount to a clever-clever instance of pop eating itself — do contemporary TV shows exist in the postmodern world of period TV spinoffs? — but it also speaks to a less arch, more affecting form of social insecurity: the terrible moment every 20-something faces when he realizes that he just doesn’t get kids anymore.

With Hill and co-writer Michael Bacall’s script swiftly reaching the conclusion that the demographic has done a lot of unwarranted growing up in the last couple of decades, the defiantly R-rated “21 Jump Street” is determined to prove that adults can do reckless immaturity rather better than kids these days. The teenagers here, led by the slippery, tidily groomed Dave Franco, are big on tolerance, eccentricity, environmental issues, insipid pop-folk music and sophisticated designer drugs — all, save the last one or two, ostensibly positive developments that the Generation X-ers behind this weirdly inverted teen movie see as far juicier satirical targets than the floppy haircuts and ill-cut denim of a TV show scarcely anyone has watched or thought about for 20 years. 

All of which, I admit, makes this cheerfully crass, stylistically slipshod and very, very funny film sounds a lot more calculated and self-admiring than it is. Borrowing little more than a logline from the original series — baby-faced cops enrol as high-school students to crack down on youth crime from within — the film asserts its own gently ironic comic tone from the get-go, taking full advantage of the fact that few viewers younger than Hill will know the difference, and few viewers older will care.

Hill and Tatum play Schmidt and Jenko, high-school foes turned policy-academy buddies: equally dunderheaded as patrol cops, they’re shifted to Jump Street duty, masquerading as decidedly overgrown teen brothers to bring down Franco’s smoothly run drug ring. By their own buffoonery, geeky Schmidt is forced into the role designated for Jenko, that of the athletic would-be prom king; Jenko, in turn, has to join the science-fair squad instead. The jokes, from here on out, pretty much write themselves: bald reversals fuel the lean storytelling, with comic beats determined as much by audience expectation and star personae than any writerly manipulation.

The surprise comes when neither fish-out-of-water figure is as awkwardly received as teen-movie lore would dictate: as Hill has little trouble landing the faintly boho blonde (Brie Larson) his Eminem-worshipping 16-year-old self could only dream of, Tatum finds an unexpected kinship with the four-eyed lab geeks, whose minimal social currency has stayed constant since 1999, as his has plummeted to their level. “21 Jump Street” isn’t the first commercial film to observe the 21st-century realignment of adolescent cliques — indeed, the 21st-century realignment of cool — but it’s the first I’ve seen to question it quite so brazenly. 

That the film skirts smugness and condescension (though not some regrettable missteps into incongruous and dissonantly nasty gross-out humor) is partly attributable to the light directorial touch of Lord and Miller, whose last effort was the scrappy animated feature “Cloudy With a Chance of Meatballs.” Treating proceedings as scarcely more credible or gravity-bound than a picture-book fantasy, they set up and shoot down the gags with necessary pace if not too much finesse, coming only temporarily unstuck in the choppy action sequences that dominate the film’s latter half.

It’s the star duo, however, that turn this iffy proposition into something first serviceable and finally a little bit special. Hill, a few pounds lighter but affably roly-poly as ever, is more subdued than his apprentice work in less specialized teen films than “Superbad” might have led us to expect, but is sparked into life by his buoyant and sincerely sweet chemistry with Tatum — who unexpectedly emerges as the comic sensation of this entire dippy enterprise.

After a long spell of quite literal grunt work that required him to act mainly from the abdomen outwards, Tatum seems finally to be waking up to the witty properties of his cartoonishly lantern-jawed hotness, as he wryly deadpans his way through a character that, for a change, isn’t any smarter than he looks, but is winningly comfortable with that. (“Fuck science!” he yells elatedly at the climax of the film’s single most riotous set piece.) With his spacily precise verbal timing and surprisingly animated physique, Tatum proves that off-the-wall comedy may yet be the best place for a lunk like him — which is one feat of reverse-type engineering on which this delightfully silly diversion couldn’t quite have counted.    

For more views on movies, awards season and other pursuits, follow @GuyLodge on Twitter. 

Sign up for Instant Alerts from In Contention!