B-

“Step Up” is based on an idea by Duane Adler, who also received story credit for “Save the Last Dance,” and there’s just a smidgen of similarity between the two movies. The plot involves two kids from completely different worlds who are linked by their love of terpsichore, even though she’s a rich kid who’s into ballet and he’s a foster child and street tough who’s more the hip-hop type. But as it turns out, they make beautiful moves together. Sound a bit familiar?

Channing Tatum (“She’s the Man”) plays Tyler, who’s busted for vandalizing the Maryland School of the Arts in Baltimore with his buddies Mac (Damaine Radcliff) and Skinny (De’Shawn Washington)–with whom he often steals cars for the local chop-shop–and sentenced to community service at the school as his punishment. While doing maintenance work there he catches the eye of Nora (Jenna Dewan), as she does his. Nora’s preparing her routine for the senior show, a good showing in which might win her the college scholarship she needs–until her partner hurts his leg. With little other option, she accepts Tyler’s offer to step in for the injured fellow, and the combination of his natural ability, her trained talent and their vastly different styles results in something special. And not just on the dance floor: after Nora breaks up with her posturing boyfriend Brett (Josh Henderson), the heat their rehearsals generates blossoms into romance, too.

As in studio pictures from the thirties and forties, there’s a supporting couple on hand here as well. They’re Lucy (Drew Sidora), Nora’s closest chum, and Miles (Mario), a combination DJ-music arranger who gets stiffed by Brett and becomes pals with Tyler. Of course, Tyler’s new relationships at the Arts School cause problems between him and his old friends Mac and Skinny, who see him as a sell-out, especially when he approaches the principal (Rachel Griffiths) about the possibility of transferring there. And a split between him and Nora threatens their routine (and their romance), until at the last minute…well, you know.

It’s Griffiths’ air of arch bemusement as she struts her authoritarian stuff that clues the audience in to how they should react to a slick piece of after-school-special material like “Step Up.” She seems to be suppressing a smirk when watching Tyler’s white-kid-pretending-to-be-hard-ass-black bit, and registers her incredulity at the implausibility of the plot turns with the slightest raising of the eyebrows. That’s the attitude you should assume when a character appears early on who’s so obviously destined for an unhappy end that he might as well have a sign reading “Dead Meat” affixed to his shirt. And he’s only the most extreme example in a script that leaves no cliche unturned in its efforts to raise spirits and tug at heartstrings.

And yet the movie works surprisingly well, because its spirit is so high and the cast so winning. Tatum demonstrates once again an engaging personality, and he can even pull off scenes that might have turned the stomach. A throwaway sequence in which Tyler interacts with two of his foster-siblings in their back yard has unexpected charm. (It helps that his “sister” is played by Alyson Stoner, the rare child actor who can actually be ingratiating rather than irritating.) And his dancing is impressive, too–no Kevin Bacon “Footloose” cutting here. While a bit stiff in the dialogue scenes, Dewan makes a good partner for him, and the other youngsters handle their stock parts nicely enough–though Washington makes rather heavy going of the antsy Skinny. Choreographer-director Anne Fletcher is predictably more successful with the dance interludes than she is with the intervening expository material, but that’s undoubtedly why she was hired.

In terns of movies involving ballet, I much prefer those (like Robert Altman’s “The Company”) that don’t suggest that the dance must be made relevant to a modern audience through an infusion of moves from the hip-hop world. It’s a message that demeans a great art form. But if you’re into this sort of thing (and the phenomenal success of “High School Musical” suggests there are lots of you out there), “Step Up” is, though formulaic and rather silly, obstinately hard to dislike, mostly thanks to its boundless energy and its agreeable cast.