WHEN Cornelia Ravenal was a student at National Cathedral School for Girls, she once was hauled into the principal's office for walking barefoot on the lawn. Her mother, while picking her up after school that day, heard about the offense. She "yelled" at the prinicpal, her daughter remembers, charging that such arcane restrictions would stifle the child's creativity.

Mrs. Ravenal needn't have worried. Her daughter Cornelia, now 25, has not only written, composed and even acted in a successful small musical -- the current New Playwrights' Theatre production "Out of the Reach of Children" -- but also has had a one-act play filmed. On top of that, Cornelia's proposal for a trade paperback is on the verge of being accepted by a publisher. Although still in the "starving" class of artist, she has ambition, energy and talent. Furthermore, she's gorgeous--frequently brushing a thick mane of curly hair out of her face with a consciously dramatic gesture.

Her mother, Carol M. Ravenal, teaches art history at American University and is a painter; her father, Earl, teaches political science at Georgetown University and is a violinist. Cornelia and her brother and sister were encouraged, to put it mildly, to be creative.

"We were compulsively creative," she said. "Our parents always made sure that we were active and productive, always achieving something." She composed and performed her own songs in school talent shows, and took dance and music lessons; her siblings both painted pictures. "The other day my father asked me to get him some music paper because he was composing a concerto. When I mentioned it to my mother, and said how great I thought it was that he was doing that, she said 'my dear, he's competing with you!' "

Their parents also encouraged self-sufficiency, leaving the Ravenal children on their own for periods of time while Mom and Dad traveled, and refusing to do much of the chauffeuring that seems to be de rigeur with children over the age of 10. Cornelia also believes she was perhaps the only one in her class of overprivileged girls at NCS who had a part-time job and paid for her own incidental expenses.

She drew on her high school career for "Out of the Reach of Children," which chronicles, largely in song, the developing personalities and friendships of five girls in a private school. The characters are composites of Ravenal herself and friends of hers. "I was more Ellen (the pudgy jokester) in elementary school. Then I lost a lot of weight and was more Marian (the artistic sexpot) in high school."

The play went through several versions while Ravenal was at Harvard, where she also met her invaluable associate Marc Johnson, who wrote all the arrangements. After graduation she spent a year here, and got a brief job acting in "Dear Desperate" at New Playwrights'. In proverbial show biz fashion, nobody would read her show until one late night when she stayed after the performance to practice some of her own songs on the theater piano. Director Harry Bagdasian happened by. "Why haven't you told me about these?" he said. "You've had the tape for five months!" she responded. The show was later given an NPT workshop, and then the current full production.

Meanwhile, Ravenal moved to New York. "I felt I wouldn't be taken seriously in Washington until I'd gone to New York," she said, and still believes that to be true. She has written two other plays, one an anti-nuclear piece for children, the other a political metaphor called "Foreign Affairs." Neither has been produced.

The casting for "Out of the Reach . . . " is strictly local. Director Fredric Lee had worked with both Melissa Berman and Caron Tate and wanted to use them, even though Berman's character originally was written as a blonde and she is dark-haired. Veanne Cox, the sultry Marian, is 19 and couldn't sing, but had such presence that they cast her. Bev Sheehan auditioned with her dog and her husband watching -- but caught the right feel for the class grind. And Gretchen Weihe not only sang well and looked right--but she went to NCS, too. And, "I've known her since I was 8," said Ravenal.

The show has been extended through Jan. 8. Ravenal, after a brief stint filling in for Tate while she was sick, has returned to New York to work on her trade book proposal, "How to Act Like a Grownup." It's a how-to manual for people whose primary piece of furniture is a stereo and who still put up posters with Scotch tape.