THERE'S SOMETHING miraculous about the American College Theater Festival, and not the least of its miracles is economic.

The 15th of these Kennedy Center fortnights begins tomorrow, and though costs have risen well over 300 percent in these 15 years, ACTF manages to be doing more on roughly the same budget.

How come?

Producer David R. Young has an answer: "We have $10 million in donated time."

Most of this is barely visible from Washington, where we see only the tip of a national iceberg. But the process begins at more than 400 institutions and continues at regional festivals in places like Emporia, Kan., Spokane, Wash.; Fort Worth or Memphis. This year 421 institutions competed for the seven places in the Washington festival.

This year's most important development is creation of a new acting company chosen from this year's performers and technicians. Some 350 seniors and graduate students are being auditioned for the 15 to 20 slots in the group.

Getting together in early summer at the Chautauqua Institution's theater school, this group will be going into rehearsals for three productions: Christopher Durang's "A History of the American Film," George M. Cohan's "The Tavern," and "El Grande de Coca-Cola," the zany revue with musical arrangements by the Folger's director, John Neville-Andrews, and Alan Shearman.

Though non-Equity, the company will be working at close-to-scale for a bus and truck tour of 26 weeks.

"In the theater," confesses Young, "we train people for a career that almost isn't. This venture will give the most talented young players we can find a chance for their first professional jobs."

It goes back to how the festival came into being.

Peggy Wood, a star who'd begun in opera and went on to leads in "Maytime," "Bittersweet" and "Blithe Spirit" as well as TV's first long-running series, "I Remember Mama," also took guest-artist roles in campus productions.

She'd come back from these engagements in a glow of professional and patriotic fervor: "People don't realize what's going on in America's college theaters," she'd lecture anyone she could buttonhole. "We're developing marvelous talents out there and something's got to be done about giving them a chance. We in the professional theater have to develop these talents."

So, on her own, Peggy Wood, who then was chairman of ANTA, the American National Theater and Academy, set out to create what didn't exist. Her ANTA associate, Roger L. Stevens, by then President Johnson's arts adviser, took to shuddering every time Peggy hove in view. He knew what she wanted.

When the Smithsonian's S. Dillon Ripley began to listen, Stevens joined up, too.

So it was that the first festival was initiated on two stages, one at Ford's, where Los Angeles City College played Congreve's "The Way of the World," and other in a tent at the Smithsonian, where Hofstra University acted Fay and Michael Kanin's "Rashomon."

Several decisive events came from that spring night of 1968. The Kanins were on hand for the "Rashomon" production and were bowled over by its competence. But these were all familiar plays. Michael Kanin urged: "Why not go further? Why not find new plays?"

So, in 1974 began what has become one of the most popular, fruitful features of the series, the playwriting awards. This year brought 82 new scripts to campus stages.

Through Kanin's wide influence, winners are given cash awards, representation by the William Morris Agency, full membership in the Dramatists Guild and publication of the play by Samuel French. From the American Theater Association's University and College Divisions came a $1,000 award to the theater department that produced the new work, substantially encouraging production of new plays.

This has led to three other playwriting citations, the Norman Lear, David Library and Lorraine Hansberry awards. Judi Ann Mason, winner of the Lear and the Hansberry in different years, worked on Lear television productions for three years and now is associated with author Alex Haley. David Library winner William Mastrosimone's newest work is "Extremities," which Susan Sarandon has been playing this year in New York. Jim Leonard, whose "The Diviners" won the ACTF XII playwriting award, saw his work go on to New York, Chicago and Los Angeles productions and is the author of the New Playwrights' Theatre's current "And They Dance Real Slow in Jackson," another ACTF winner.

Kanin's contributions have been incalculable. He and his wife, now president of the Motion Picture Academy of Arts and Sciences, have tapped their associates to "take an interest." Now there also are musical theater awards from ASCAP (though this year there is none in that category) and for design excellence; internships in England through the British-American Repertory Company and for the O'Neill Center's National Critics' Institute. For the first time this year, the Kanins will not be in the festival audience, for he had a heart attack this winter at his Santa Monica beach house.

The Kanins' interest also sparked another aspect of the festival that has attracted agents from TV-land: "Irene Ryan Night," devoted to winnowing out performing talents. "I know we're aimed at theater," says Kanin, "But TV is a part of theater most hungry for fresh writing talents."

Irene Ryan, a vaudeville veteran appearing at the Kennedy Center in the 1972 musical, "Pippin," was taken with the idea of the festival. With no family of her own, she left her considerable earnings to ACTF for "promising young performers" to continue their studies. This created a concurrent 12-region competition, with the latest batch of regional finalists to be judged next Sunday evening with Peter Falk, now appearing in "Make and Break" at the Eisenhower, as the evening's notable.

Ahead lies another possibility, cable TV. With a burgeoning field literally eating up material, striving youth is an obvious source of fresh viewpoints. As our increasingly impressive regional theaters have been proving, Peggy Wood was right. We have a lot of talents waiting to be plucked.