On Tuesday night at the Peter Jay Sharp Theater five teenage girls from the School of American Ballet, dancing to Tchaikovsky, each extended a leg toward the audience above hip height. Then, keeping the legs high in that position, they all began to fall forward off balance toward the audience. They didn't actually fall, of course, but, before they brought the legs down, they had transferred much of their weight in its direction. It was easy for the audience to feel the suspense involved in the moment of extension and then the audacity involved in that transfer of weight: those girls were aiming those legs straight at us. The work was George Balanchine's ''Allegro Brillante'' (1956). But though New York City Ballet and American Ballet Theater have danced this in recent years, they never made that moment so clear. Whereas on Tuesday these five girls proclaimed it as a hallmark of the style in which they have been trained -- as if to say, ''School of American Ballet.'' The school, which Balanchine founded in 1934, has been presenting annual workshop performances now for 47 years. Traditionally there are three performances at this time of year; Tuesday's was also the benefit. [ABSTRACT FROM AUTHOR]