THINK you can write a radio script? Seven third-graders at Phyllis E. Williams Elementary School in Upper Marlboro thought they could, and did.

On Saturday, at 10 a.m., their play, "Mercury, the Fastest Kid in Town," will be broadcast live via satellite to National Public Radio stations across the country. Theirs is one of four scripts to win the Children's Radio Theater's eighth annual Henny Penny Playwriting Contest. The radio productions are part of the 10th annual Imagination Celebration festival at the Kennedy Center from Friday to April 29.

"Mercury" is based on a book the youngsters wrote last year, as second graders, while they were studying mythology. That book was so good it won Prince George's County's annual Write-a-Book Contest.

And if that seems impressive, add this: A script by the school's fourth graders, "How We Got Snowflakes," was one of eight honorable mentions in this year's Henny Penny competition, which drew 1,900 entries from students 7 to 17 in 47 states, Canada and Europe.

And this: a group of fourth-through-sixth graders from the same school also won the first Henny Penny contest.

This Friday at 12:30, the seven young scriptwriters -- Evelyn Gilmore, Ashley Lumpkins, Aleeza Kersey, Darryl Peters, Joshua Miller and Toby Andersen -- plus the fourth grade authors of "Snowflake," their classes and parents will attend the premiere performance of "Mercury" and the three other winning plays: "Word Traveler" (a girl and an aardvark journey through a dictionary) by Marisa Kantor, 12, of Matawan, N.J.; "Syntax Error" (a teen-ager's disastrous blind date arranged by computer) by Cecily Anne Schoen, 15, of Chicago; and "A Play Extempore" (a mystery based on Shakespeare's "Henry IV") by Anne Barthel, 15, of New York City.

Actress Tina Yothers (Jennifer in NBC's "Family Ties") will serve as guest host and provide continuity for the four productions. Six professional actors (including Kevin Davis, 12, and Erika Bogren, 16) will read all four plays. The first public performance is scheduled for 7:30 Friday night. Saturday morning's broadcast will be carried locally on WPFW-FM (89.3).

Elizabeth Wildberger, library media specialist at Williams Elementary, is the mentor behind both the school's success, supervising the first winning script as well as "Snowflake" and "Mercury." This year she and reading specialist Joan Tetrault worked with the young scriptwriters, all in the school's talented and gifted program. They call themselves "The Crackerjacks."

Wildberger left her job with Maryland Public Television, where she worked on two series that presented programs based on children's books, to join the staff at the school when it opened in 1976. "We do a lot of television productions in school. We have our own in-house broadcasting -- but we don't produce it unless" the children script it, she said.

The book "Mercury" evolved when the youngsters were studying mythology. "We decided to think about what the gods and goddesses were like when they were little kids," said Wildberger. "We used brainstorming -- we teach them from kindergarten on that everybody's ideas are good, that you can piggyback on another idea; then we sort of prioritize them and evaluate what makes a good story. We generally come out with something strong. It's a practice that works for us."

In the script adaptation, the fleetfooted Roman messenger cleans the house in seconds, captures robbers singlehandedly and winds up on Mount Olympus.

This area seems to produce more than its share of young playwrights. Among last year's five winning plays were "Mud Monster" by 10 second graders from Elkridge (Md.) Elementary School; "How It Came to Be" by 12-year-old Bernie Liu of McLean, a student at Haycock Elementary School in Falls Church; and "Galaxy Girl and the Great Nuclear Nemesis" by 13-year-old Donna Mulvihill of Bethesda, a student at Little Flower School, who marked her third win in the Henny Penny contest -- a first place win in 1982 and an honorable mention in 1983.

All three -- plus "A Dagger of the Mind" and "The Light of Truth," the other 1985 winners -- are available for $8 on cassette (as "Children's Playhouse 16") from Children's Radio Theater, Box 53057, Washington, D.C. 20009.