THE D.C. Youth Orchestra, which gave its 20th anniversary concert at the Kennedy Center the other evening, is a model of a successful public school effort. No make that a model of success for any school. To its home at Coolidge High School it draws, from across the metropolitan area, some of the best young musicians from public, private and parochial schools alike. New musicians are taught how to play -- 8-year-olds start on half-size violins -- and then compete to qualify for orchestras and wind ensembles on different levels.

And it works. The whole caboodle, with its 800 youngsters, not only make good music but makes music in the midst of all the political, racial and financial discord that invites it to fail. On Saturday mornings, the Coolidge parking lot is crammed with D.C., Virginia and Maryland cars filled with people of all colors, with people both rich and poor.

When the orchestra begain in 1961, desegregation was prompting a white-to-black shift in the District schools. "People told me no one will support an all-white orchestra," says Lyn McLain, the orchestra's only director in its 20 years. "I told them it wasn't going to be all white, and they looked at me like I was crazy. They said blacks can't play classical music. Well, we got started. If you can play better than the first violinist, then you've got the seat. That's the only rule we've got. No racial quotas, and there are blacks and whites here. I call it natural integration."

The Youth Orchestra has won world acclaim for performance on a professional level. It has played in St. Moritz and Berlin, in England and Japan. In 1980, the Organization of American States asked it to record the works of contemporary American and Latin American composers. In August, the orchestra will go to Israel and Greece, with money for the trip coming, as usual, from local performances, patrons and parents.

The youth orchestra represents the best of the D.C. schools.' It works for the whole Washington community. No matter what financial catastrophe may come to the city's public schools, this orchestra should not be sacrificed. Encore!