An 18-year-old Prince George's County youth who has written a musical and exhorted his classmates with the words of the companion of a six-foot-tall imaginary white rabbit was recently named a Presidential Scholar.

Stephen L. Hayes, 8028 Carey Branc PLace, Oxon Hill, was one of 121 scholars selected on the basis of high college board scores and interviews with a presidential panel. He's considered one of the brightest high school graduates in the country.

Hayes wrote the book and lyrics for a musical entitled "Evening Final." A friend wrote the music, and the script won a competition run by Georgetown University's Mask and Bauble Society and was produced by the society. The musical concerned a newspaper reporter and its complex plot involved flashbacks about the reporter, saving until the end the revelation that she had died.

"Evening Final" was scarcely Hayes's first entry into the dramatic world. He played leading roles in many high school productions, including that of Elwood P. Dowd in the play "Harvey."

Hayes quoted from Dowd in his valesictory speech at Gonzaga High School in Washington, telling his classmates: "My mother always told me that 'In this world, Elwood, you've either got to be oh-so-smart or oh-so-pleasant.' Well, for years I was smart and I recommend pleasant."

While at Gonzaga, Hayes played Tevie in "Fiddler on the Roof," Riff in "West Side Story," Cornelius in "Hello, in Business Without Really Trying,"

He is working this summer in downtown Washington for the National Solid Waste Management Association but is rehearsing at night for a late July openinig of "Cabaret" at new dinner theater in Oxon Hill."

Hayes plans to attend college at Yale. He is interested in writing or performing for theater, but thinks he might also want to do general writing.

Another Maryland youth, Steven C. Reber, 17, of 7006 Old Cabin Lane, Rockville, was also selected as a Presidential Scholar.

Reber is a graduate of Charles C. Woodward High School in Rockville and was one of several valedictorians for his high school class. He plans to attend M.L.T. and specialize in electrical engineering.

Reber left for a two-month vacation in Europe immediately after the Presidential Scholars were given their awards, and could not be reached. In the essay he submitted to the awards panel, he cited an arrangement by which he took college courses at Montgomery College while in his senior year at Woodward High School as one of his best educational experiences.

ThePresidential Scholars program began under President Lyndon Johnson as a means of recognizing outstanding high school graduates. Scholars receive a medallion, but no monetary award.

Candidates were selected first from college board scores, and the finalists were chosen after interviews by a presidential panel. The scholars "had to measure up in terms of total personality, with outside activities and exposure to the arts and humanities as well as science," according to Edith Roth, a spokesman for the U.S. office of Education, which runs the program.

A District of Columbia youth, Christine A. Desan, 18, was also chosenas a Presidential Scholar.