On the softly lit stage of the Kennedy Center Terrace Theater, 48 young voices swing into a snappy Duke Ellington medley as their enchanted audience glows with admiration. Parents and grandparents, brothers and sisters beam as the youthful choir, clad in tuxedos and black gowns, conquers the exalted tones of the Schubert Mass in G and moves into the catchy lyrics of the Appalachian folk song, "Cindy."

Only a few hours earlier, this sophisticated singing troupe, the D.C. Youth Chorale, was a giddy group of high school students chattering about final exams, joined by a fidgety crew of elementary school pupils wiggling about as their exasperated conductor tried to hammer in just the right rhythm for a solemn rendition of "No Man Is an Island."

About once a month, the D.C. Youth Chorale members are transformed from high-spirited students to polished performers as they travel with their first-rate music from rehearsal rooms in local schools and churches to stages in New York, Philadelphia and even overseas.

The youth chorale is the city's elite student chorus and on Sunday afternoon the group celebrated its 20th anniversary at the Kennedy Center with a showcase of the chorale's past and present talent. Last Friday Mayor Marion Barry proclaimed May 17 "D.C. Youth Chorale Day," commending the chorale for its years of serious music instruction for the city's youngsters.

From as far away as Colorado, alumni came to sing with the present chorus in a celebration that was largely a tribute to the chorale's founder, Frances W. Hughes, and its director since 1966, Edward Jackson Jr.

"Frances Hughes and the youth chorale are synonymous," said Wilma Shakesnider, one of the chorale's most renowned graduates who has performed in opera productions around the country. Shakesnider, a soprano who graduated from Roosevelt High School, starred in the Houston Opera's long-running production of "Porgy and Bess" and now teaches at the University of Colorado. She returned as a soloist Sunday, she said, because of her high regard for Hughes, who guided her in her career.

Hughes, a dynamic woman in an elegant print gown, dreamed up the idea of the youth chorale during the 1950s, when she decided the city's school children were poorly trained in music. A former Howard University music instructor and later a teacher and principal in the D.C. public schools, Hughes envisioned a program that would expose children to the creative arts and equip them with skills in music, drama and dance. When she approached school officials in 1961 with her idea, she recalled, she told them, "General music classes take care of music for the millions. But what about the talented few?"

From the 130 pupils accepted in the original group, Young Scholars with Special Gifts, the after-school program has expanded in two decades to include about 200 members in a senior high school chorus, opera workshop, elementary-junior high school choir, alumni division and the Zuri Watu dance-instrument troupe. The program's latest addition is the Chorale Connection, an adult division, and on stage Sunday at the Kennedy Center, several gray heads stood out among the pigtails and ribbons of the younger singers.

While Sunday's performance was an opportunity to show off some of the chorale alumni who have been successful in the performing arts, many former members who now are computer programmers, police officers and teachers also joined in, mainly out of affection for Hughes and Jackson.

"This is my way of trying to contribute," said Earl Davy, a chorale member from 1969 to 1971 and now a Secret Service agent, who was asked only last week to sight-read Schubert's Mass with the senior division on Sunday. "I can't pull too much money out of my pocket, so I'll give them the time."

Many of the alumni agree that the chorale's travels around the globe were one of its greatest benefits. "Have group, will travel" has been a constant motto, Jackson said, and that theme has taken the chorale to Boston, New York, Philadelphia, Chicago, Mexico and Romania, as well as to numerous stages around the Washington area.

Lately, the chorale has fallen on hard times financially. Its budget, provided by the public schools, was as high as $79,000 in 1976 but has been reduced to about $50,000 annually, said Jackson, a soft-spoken man who inspires deep loyalty from his students. The program also has suffered from cuts in the number of music teachers in the city's schools, who in the past served as talent scouts for the youth chorale.

With the support of a thriving parents group and the energetic leadership of Hughes and Jackson, the youth chorale still attracts the city's finest young voices to after-school and Saturday morning rehearsals. And the group also has managed to stay on the road, partly with financial backing from Temporaries, Inc., which Jackson calls "our corporate angel."

Backstage last Sunday, watching the chorale alumni perform and waiting for his turn to sing as first tenor, Anthony Jackson, a gregarious junior at McKinley High School wearing oversized horn-rimmed glasses, expressed the essence of the youth chorale: "It's preparing me to do what I really like, to become a better musician. I give up a lot of time for it, but it will pay off in the end. Maybe some day I'll go to Broadway."