It was upbeat all the way. Clean, young, idealistic and enormously talented, America's 20 Presidential Scholars in the Arts put on a dazzling display of dance, music, acting, literary readings and visual arts at the Kennedy Center last night for the 121 other scholars, parents, government officials and the public.

Nobody wanted to talk much about the flap over the announced plan of one of the scholars, Ariela Gross of Princeton, N.J., to give President Reagan a petition favoring a nuclear freeze at a White House reception this morning for these top high school students from across the nation.

"The press is elaborating on it too much," said Debra A. Seddon of Rockville, a painter and sculptor, backstage before the performances.

"The reality is that something very nice and very positive is being done here," said Kevin A. Berlin, a painter from Potomac.

"I'm not well informed; I don't know anything about it," joked Andrew C. Pearce, a cellist from California.

"I thought everybody from California was beachy keen," Nicholas J. Kitchen, a violinist from North Carolina, shot back.

The brief performances brought strong and sometimes thunderous applause. Violinist Timothy P. Ying of Illinois performed Pablo Sarasate's Introduction and Tarantella with a tone so sweet and a control so perfect that it brought down the house.

Short-story writer and poet Rachel J. Pastan of Potomac read her story "Salamanders in Long Grass," about a little girl who tells her baby sitter there is a salamander inside her. After she goes to bed, she gets up to see her mother and father returning home in their chariot, moonlight playing on her mother's silver dress. In the morning, she lets the salamander come out and releases it into the long grass.

"It's about fantasy . . . what people do to escape," Pastan explained before her reading. She said people need to escape from "the fear of death and the basic boringness of everyday life." In the fall she is going to Harvard.

George W. Dick of Washington, an actor, played a powerful scene from "Candide" as adapted by James MacTaggert, then switched for a second performance into the role of a street tough from Charles Fuller's "Zooman and the Sign."

The visual artists, including Berlin and Seddon, talked about their general philosophies and how they create their work while slides of their work were shown on the wall behind them.

"I have become sensitive to nuance," said Berlin, who does nudes and other strong representations of human figures and faces. " . . . Living models are my favorite objects." There was some laughter, apparently because a slide showing a nude female came on just at this time and Berlin's tone was super-serious, but this soon died down and Berlin won over the audience with his passionate devotion to art.

"My speaking here tonight proves that through hard work and determination the American dream is as valid as ever," Berlin said. He received strong applause.

Seddon called art "a window to the inner soul," and said that as a child, "my art objects were my toys and fantasies."

At the end of the performances, each of the Presidential Scholars in the Arts--chosen from more than 5,500 student artists who originally applied in a talent search conducted by the National Foundation for Advancement in the Arts, a private organization--received a $1,000 award.

Actress Cicely Tyson and baritone and actor William Warfield gave the youngsters words of encouragement.

Said Tyson: "While I was sitting there watching, I thought how fortunate you are to know at this stage of your lives what contributions you would like to make to humankind."