Of all the things to bring to big-city elementary schools as an elective program ... ballroom dance? It's a crazy idea, and I'm so glad someone did it, because it leads us to "Mad Hot Ballroom," an exuberant documentary that follows three New York City schools as they prepare for the annual city-wide competition.

Documentaries that are this much fun and this full of innocent energy are rare. Director Marilyn Agrelo and her crew hit on a near-perfect situation in that the ballroom program is for fifth-graders, so the kids are old enough to be developing distinct personalities, but not so old that they're trying to act like grownups. A movie about 8-year-olds or 12-year-olds who dance would not, I think, have been nearly as amusing as one about 10-year-olds.

We meet the kids of P.S. 115 in Manhattan's Washington Heights, predominantly Dominican, with 97 percent of the kids in school living below the poverty level. Downtown in Tribeca, there's P.S. 150, where the ethnicities (white, black, Hispanic, Asian) are more mixed. Over in Brooklyn, at P.S. 112, we meet a lot of Asian and Italian students. This is all in stark contrast to last year's defending champions, Queens' P.S. 144, where everyone seems very white and very Jewish. If nothing else, the movie reminds you of what a melting pot New York is.

Led by professional instructors who work with the teachers, the entire fifth-grade classes take ballroom lessons, with six pairs -- five competitors and an alternate -- eventually being chosen to dance in the tournament. The kids learn everything, but the competition focuses on the rumba, tango, foxtrot, swing and merengue. The winner gets a huge trophy, taller than most of the students. It currently resides with those smug jerks in Queens. It is highly coveted.

The dancing scenes are endlessly entertaining, especially early on, when the boys are still getting used to dancing with girls and when even the most awkward students earnestly try to get the steps right, bless their hearts.

But Agrelo doesn't stop there. She follows the kids as they walk home from school, play foosball, loiter on playgrounds. We learn what they're like. The girls are mostly more mature, usually taller than the boys. They're starting to notice boys and to think about serious things. They talk about how they would never date someone who was a drug dealer, and how they want to grow up to have happy families. The boys, meanwhile, act silly and talk about how bossy the girls are. If anyone ever dismisses ballroom dancing as being for sissies, it isn't shown. For these boys, it's just fun, that's all.

There is heartbreak in the quarterfinals, and then a champion team emerges -- good thing it happened to be one of the schools Agrelo was following! -- that we can root for in the semifinals. And I note the same air of suspense I felt during "Spellbound," the doc about the national spelling bee: Since this is a documentary, limited to reporting what actually happened, there is no assurance that the people we like will win.

Much is made of how most of these kids come from single-parent families, some of them with behavioral problems in the past, and how the dance program gives them something to look forward to, whether they're any good or not. And you can see it in their cherubic little faces, the joy of doing something interesting, of being with their fellow students, of earning praise from adults. Too many lousy movies have been made about the transformative power of dance; in "Mad Hot Ballroom," you understand perfectly how the music and the rhythm could change someone's life for the better.

Grade: A-

Rated PG, very mild thematic material

1 hr., 50 min.