THE teen dance drama “Step Up” seems like it was not only inspired by a Janet Jackson video but entirely written during one.

As a petty criminal with hip-hop moves (Channing Tatum) and a rich ballerina (Jenna Dewan) find common ground in the dance studio, the movie is so pleased to be vacuous that you will at least smile during its pumped-up dancing scenes. It doesn’t bore you with the socioeconomic pieties of last spring’s similar “Take the Lead.”

Tyler is a street tough who, after breaking into and trashing an exclusive Baltimore high school for the performing arts, gets sentenced to 200 hours of community service – as a janitor at the school he just vandalized. Would the school really want this guy around? Does Cartier award internships to jewel thieves?

The school is overrun with talent – in the hallway, two dudes in cornrows are playing Pachelbel on their violins – yet Nora, the hottest ballerina, can’t find a dance partner for the big senior show she’s choreographing and starring in. So she picks the new janitor, who once busted a couple of hip-hop moves in the parking lot while she peeked through a window. “I can’t believe there’s no one else,” says the head of the school (Rachel Griffiths). Me neither. Nor am I clear on why a school project is open to nonstudents (what do they do at the homecoming football game – bring in Peyton Manning at QB?); or why a slouchy-pantsed rebel would take a sudden interest in pliés, barres and that thing where you wiggle your feet while leaping in the air. Hormones, you say. Fine, but the movie forgot to put in the passion. The couple smooch a little but keep their moves strictly vertical.

“Step Up” is so predictable, with every scene loudly announcing what the next one is going to show, that teens in the audience will take lots of breaks to text each other on their phones. A couple of subplots are seriously underfed. Tyler’s supposed rival, Nora’s boyfriend, is on-screen so briefly that we don’t even get to know him, let alone hate him.

The blocky Tatum (ears by Prince Charles, facial expressions by Rocky Balboa) is likeable enough, and Dewan, who looks like Paula Abdul Jr. (and has danced with Janet Jackson), is a honey with more cute outfits than the September issue of Lucky. The actors do their own dancing, which makes their routines bright and convincing. There’s none of that cheesy “Flashdance” fakery (close-up on her face! Close-up on her feet! Now we see her whole body but the light is so poor that you can’t make out her face!). Easily the best scene in “Step Up” is one in which everybody goes to a party and busts out dancing to an R&B mashup.

But when they talk, ouch. “I’m fighting for something that’s real for the first time in my life.” ” ‘Sorry’ is not gonna cut it!” Griffiths, who spends the movie standing around frowning, sounds like she just got back from a weekend-long shopping spree at the Cliché Costco: “A lot of students have talent, but it takes so much more than that . . . the only kids that make it are the ones prepared to fight for it . . . show me that you want it, really want it.” All of this, in one speech. Can it be only a year since this woman was playing Brenda on “Six Feet Under”?

“Step Up” asks us to believe that a girl would dump a guy who just scored a record deal. That ghetto car thieves say things like, “You must have all the recessive genes in the family.” And that a ballerina would design a dance number in which the guy gets as much attention as she does. “This whole thing is stiff,” says Ty, who is referring to either a specific dance or the entire movie. “It’s boring.”

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STEP UP

[**] (Two stars)

Shut up.

Running time: 98 minutes. Rated PG-13 (profanity, brief violence). At the Lincoln Square, the Orpheum, the Kips Bay, others.