When George Balanchine was invited to come to the US to start a ballet troupe, he replied--so legend tells--""but first, a school."" The line makes an apt title for this chronicle of the School of American Ballet, for 50 years a symbol of the late Russian choreographer's dream to establish a dance culture, on these shores. In a time when ""American Ballet"" was a contradiction in terms, the odds against Balanchine, his American sponsor Lincoln Kirstein, and their colleagues seemed formidable: ""Serious, disciplined ballet classes in New York in the 1930s were not just rare, but served mostly adults,"" Dunning writes. Nevertheless, the school opened its doors to 32 students in January, 1934. And despite temperamental Russian teachers, rapidly bankrupted benefactors, and classroom mirrors that didn't hang straight, it survived and grew. Today, housed in spacious quarters near Lincoln Center, the School of American Ballet annually spends $1.4 million to train 350 students from all over the world. A New York Times dance critic, Dunning successfully avoids the adoring PR tone that often suffuses institutional biographies. She also keeps her focus firmly on the school, instead of straying into a history of its sister establishment, the New York City Ballet (no mean feat, since the corps sprang--literally--from the school's ranks). Unfortunately, Dunning's limited not only her subject, but her approach as well: she's clearly done exhaustive research in the archives, but disappointingly few flesh interviews--most of the first-person reminscences are quotes from previously-published books or articles. As a result, the book seems disjointed, lacking in vivid description until it reaches modern times. And even then, Dunning's flat prose provides little in the way of effective atmosphere or analysis: a meticulous discussion of how the Children's 1 level differs from Preparatory A1, but little sense of the school as a place synonymous with US ballet training. In sum, a Triple B rating: Basically for Balanchine Buffs.