I'M not sure who's happier at this year's Young Playwrights Festival. Is it the 13-year-old dramatist Jason Brown, who gets to see his one-act play receive a professional New York staging at the Public Theater? Or is it the audience, which gets to discover Mr. Brown's emerging talent? Let's just say that there's a lot of good feeling flying about in the Public's Martinson Hall these nights.
Now in its third year, the Young Playwrights Festival is run by The Foundation of the Dramatists Guild, under the artistic direction of Gerald Chapman. This spring's 10 winning plays - each awarded a first-class adult production - were selected from 1,160 entries submitted by authors nationwide under age 19. Five of the winning scripts are receiving staged readings (also at the Public, with free admission); the five I saw have been assembled into a single theatrical evening, complete with elaborate sets by the gifted designer Loren Sherman.
The bill is long, and, in general, not as consistently well written and acted as last year's. But the two excellent plays, Mr. Brown's ''Tender Places'' and Patricia Durkin's ''Fixed Up,'' justify the enterprise, and even the three lesser works have their moments. Mr. Brown's play, the evening's final offering, is astonishing. ''Tender Places'' is about a gifted, 12-year-old boy named Eric (Knowl Johnson) who is constantly yanked between his recently divorced parents (Carolyn Mignini and Stephen Vinovich). While this subject might lead to sentimentality or self-pity - especially in a writer this young - Mr. Brown tells his tale with humor and circumspection. As a result, he unearths the real meaning within his material. Though ''Tender Places'' begins by mocking divorced parents who use bribes (toys, movie outings) to compete for their child's affections, it is ultimately just as tough on the son. Eric tries to manipulate his parents, too - only to discover that he pays the price of losing contact with his own emotional needs.
How Mr. Brown's protagonist regains his equanimity is the story of the play. The author gets to his touching, cleverly managed resolution with bright, indirect dialogue. When Eric gets tired of hearing his smothering parents repeatedly say how much they love him, he suggests that they hate him instead - because ''maybe it wouldn't hurt so much.'' When the mother suggests that Eric see a psychologist, the son snaps back sarcastically, '' You make me nuts, and I go to the shrink!'' If ''Tender Places'' is autobiographical - as Mr. Brown, who wrote the play at age 12, seems to indicate in his program biography - it's quite likely that its wise author may never need a psychologist. What he may well need someday is an agent.
The play is expertly served by both its cast and its director, Shelly Raffle. Miss Raffle also directed ''Fixed Up,'' in which Miss Durkin writes about what may be the most misbegotten blind date in the history of high-school senior proms. Jeffrey, a brainy kid soon heading for the Massachusetts Institute of Technology, finds himself stuck with Laura, a chatty loser whose aggressive efforts to be attractive and personable all backfire.
Like ''Tender Places,'' ''Fixed Up'' could have been an exercise in self- martyrdom, and, indeed, Laura is self-pitying and self-deprecating. But again the young playwright (age 16 at the time of writing) examines her characters from an objective perspective. What begins as a witty farce about a typical adolescent mishap soon expands into a telling revelation of the heroine's first step into maturity. By the prom's final dance, Jeffrey has helped Laura move on from self- pity to self-examination - and the process is finely delineated by the lively, often funny performances of Ellen Mareneck and Marc Epstein.
The evening's opening plays, Catherine Castellani's ''Romance'' and Juan Nunez's ''Meeting the Winter Bike Rider,'' are dissections of, respectively, male-female and male- male bonds. They are both sensitively directed by Elinor Renfield, who has cast an appealing couple, Catherine Ann Christianson and Rob Knepper, in ''Romance.'' The bill's longest play is Anne Harris's ''In the Garden,'' in which Etain O'Malley and Pamela Payton-Wright (under James Milton's direction) stridently portray two squabbling spinster sisters reunited by their mother's death. Miss Harris's sophisticated script creates a tense, neurotic environment that's much more fascinating than the characters who inhabit it. But even the play's failure doesn't diminish the special pleasure of watching a young writer with talent at work.
Five Offerings THE YOUNG PLAYWRIGHTS FESTIVAL. Sets by Loren Sherman; costumes by Patricia McGourty; lighting by Mal Sturchio; sound coordinator, Bill Dreisbach; recorded music for ''Fixed Up'' by Michael Rubell and Whyte Lyte; resident composer, Louis Rosen; production stage manager, Esther Cohen; stage managers, Morton Milder and Diane Ward; producing director, Peggy C. Hansen; artistic director, Gerald Chapman. The Foundation of the Dramatists Guild's Production presented by Joseph Papp. At the Public Theater/Martinson Hall, 425 Lafayette Street. ROMANCE, by Catherine Castellani; directed by Elinor Renfield; original music by Louis Rosen. WomanCatherine Ann Christianson ManRob Knepper and MEETING THE WINTER BIKE RIDER, by Juan Nunez; directed by Elinor Renfield; original music by Louis Rosen. MarkJeffrey Marcus TonyCorey Parker and FIXED UP, by Patricia Durkin; directed by Shelly Raffle; music recorded by Michael Rubell and Whyte Lyte. LauraEllen Mareneck JeffreyMarc Epstein and IN THE GARDEN, by Anne Harris; directed by James Milton; original music by Louis Rosen. KateEtain O'Malley MaryPamela Payton-Wright and TENDER PLACES, by Jason Brown; directed by Shelly Raffle. MaryCarolyn Mignini PaulStephen Vinovich EricKnowl Johnson SamLois Diane Hicks