After a 21-year career a lot of dancers would be left asking a little forlornly: What next? Not Sara Leland. To her status as principal dancer of the New York City Ballet she has added a second title: assistant ballet master. As fluidly as she executes a solo in ''The Nutcracker,'' she is moving from the role of performer to teacher and coach.
American dancers, Miss Leland maintains, are the hardest-working in the world. ''I have seen superb dancing by 15- and 16-year-old girls in small Midwestern companies,'' she says.
As a performer, she has been in the forefront during a period of burgeoning national interest in the dance. Now, as a mentor, she wants to be a part of consolidating that interest into a major artistic tradition.
Her own mentors have been the two great names of the New York City Ballet: George Balanchine and Jerome Robbins. ''I'm watching Mr. Balanchine as much and as closely as possible these days,'' she says. ''I treasure every minute of every rehearsal that he conducts. I'm trying to learn his ballets so accurately that I will never forget them and can stage them in the future exactly as he intended.''
Some of Miss Leland's fellow dancers term her a ''memory bank'' because of her ability not only to retain and teach steps, but to remember parts and communicate the entire feeling of over 30 ballets. Mr. Balanchine has so trusted this retentive talent that he has dispatched her to many ballet companies around the world to teach their dancers his choreography and to transmit those details of style that give his ballets life. She uses videotapes to help teach, but says she believes strongly in imparting personally both the technical details and the inner depths of these ballets.
Beyond memory, beyond some skill in communication, what makes a good dancer a good teacher? Miss Leland gives a specific example: ''Right now I am helping many with the music. It is important that a dancer be musical and hear clearly what the music is saying. I flatter myself that I can hear music and that it makes sense to me, so I try to show others, who are having difficulty, how they should be listening to the music. When they learn to listen, they work more easily, more simply and logically.''
In general, her recipe for mentors as well as dancers is discipline and hard work. ''You can never stop concentrating,'' says this woman, who has been doing just that since she was a five-year-old named Sallie Harrington in Melrose, Mass. It was then she began to study ballet with E. Virginia Williams, who later founded the New England Civic Ballet, forerunner of the Boston Ballet. Her mother, Ruth Harrington, who ''ran the desk'' for Miss Williams while her daughter was a pupil, is still associated with the Boston Ballet School. Her husband, New York businessman Arthur Kervorkian, has also been entirely supportive of his wife's career.
It has been as long road from those days, through the corps de ballet, to principal dancer in such New York City Ballet classics as ''The Goldberg Variations,'' ''Vienna Waltzes,'' ''Union Jack,'' and ''Jewels.'' Miss Leland, the dancer, has no doubt that it has all been worth it for herself. Her dance career, she claims, has been exciting and artistically creative. ''You can never get away from the grueling schedule. But you have the camaraderie of great friends in the company, you always feel like a ballerina, and you have the fun of travel to far corners, including Russia and Europe.''
As mentor, she speaks more cautiously: ''Ballet is such a hard career that unless you have the talent, the physical structure, and the musicality for it, you can get caught up in it but end up going nowhere. A top teacher or ballet school can evaluate a child's potential on a frank and impersonal basis - before things go too far.''
Miss Leland says the School of American Ballet in New York conducts auditions at which ballet students can be evaluated every Wednesday afternoon from mid-September through the end of July. To parents wondering how to find a good local ballet teacher for their children, she recommends contacting officials at such professional schools as those associated with the New York City Ballet (the School of American Ballet), the Joffrey Ballet, and the American Ballet Theater, and asking for names of teachers with whom they are familiar. There is no licensing of ballet teachers, she says, and no directory, although Dance Magazine does carry a listing of some teachers.
As for the future, will the woman who has been described as a ''choreographer's dancer'' become known as a ''dancer's choreographer''? That, like so much else in ballet, remains in graceful suspense.