A FEW WEEKS ago in the Berkshire hills of western Massachusets a young conductor looked out the stage door and saw Leonard Bernstein, Seiji Ozawa and Gunther Schuller slipping into seats down front in the small theater at Tanglewood. He turned to his fellow conductor who was to open the program and said, "Harry, at age 10 did you ever think this would happen to you?"

Carl Roskott came to Tanglewood this summer to find out just how good a conductor he was. He knew that in this same place 20 years earlier Ozawa had met the same challenge. And 20 years before Ozawa there had been Bernstein.

Roskott was one of five finalists in the National Symphony's assistant conductor auditions last month at the Kennedy Center. But as he walked out onstage to conduct the student orchestra, told himself, "This is it, this is what you've been working for - you will never have a better panel of judges."

The Tanglewood vision of Serge Koussevitzy was once again renewing itself. Its foundations are inspiration and aspiration, the inspiration of the professional linked to the aspiration of the young in a setting of beauty.

That fundamental premise has altered little over the years. As Ozawa, the Boston Symphony's present director, remarked, "I'm sure that if Koussevitzy came back today he'd say, "Seiji, it's the same." There's still the performance of very serious music, the study of very serious music, the study of very serious music and nature."

Koussevitzy's genius was to form an interlocking relationship between the three elements. As music director of the Boston Symphony, he first brought the orchestra to the grounds of Tanglewood in the summer of 1936 to present the Berkshire Music Festival. Four years later, he established his school - the Berkshire Music Center, sponsored and staffed by the Boston Symphony - on those same grounds.

The partnership between the orchestra and the school has turned Tanglewood's 210 acres into some of America's most fertile musical soil. Three of this country's major orchestras are currently led by former Tanglewood students. The Cleveland Orchestra's Lorin Maazel, the New York Philharmonic's Zubin Mehta and Boston's Ozawa all passed through the Berkshire Music Center. Its annual crop of instrumental players feeds orchestras all over the country. The Boston Symphony has 42 Tanglewood alumni, the Philadelphia Orchestra 22, the National Symphony 21 and the Los Angeles Philharmonic 13, to single out just a few. A list of composers nurtured at Tanglewood reads like a "Who's Who" of contemporary music.

The remarkable success of Tanglewood is due to what Dan Gustin, who doubles as administrator for the school and an assistant manager for the orchestra, calls "a symbiotic relationship" between the orchestra and the school, "the intimate living together of two kinds of organisms, especially where such association is of mutual advantage." That describes exactly the state of affairs during the summer at Tanglewood.

For eight weeks the Boston Symphony and the student musicians are thrown together in an intense round of musicmaking. On one side of the grassy expanse known as "the lawn" the symphony rehearses for weekend performances in a 5,000-seat shed. On the other side of the lawn in a series of smaller buildings, students from all over the world study and rehearse for weeknight performances. Movement back and forth across the lawn is continuous, a symbol of the constant interchange taking place.

This season, for the first time in Tanglewood's history, another orchestra besides the Boston Symphony will play in the shed. When the Boston goes on a tour of European festivals - beginning at Salzburg on Aug. 24 - Tanglewood alumnus Zubin Mehta will bring the New York Philharmonic to the Berkshires to present the final round of concerts. (In the pre-Tanglewood days, when the Berkshire Music Festival was looking for a permanent home, it was the New York Philharmonic which gave the first concerts in the Berkshires.)

On Sunday, Aug. 26, the season will close with a performance of Beethoven's Ninth Symphony, a custom begun by Koussevitsky and still maintained.

During the 56 days at Tanglewood, what the students gain is obvious. The Boston Symphony opens itself completely to them. They have free entry to all concerts, can attend all rehearsals, and watch as Bernstein or Ozawa or Ormandy puts a program together. They can go backstage to kibbitz; they can see, as one delighted student recalled, "the members of the Boston Symphony eating doughnuts." Daily contact with these professionals changes the way a student views himself.

"You can see yourself in a development period and you can see the people who've made it," said violinist Sarah Sherry, a student from Chevy Chase. "Even though there's an enormous span, you begin to feel you have some type of place in it."

This immersion in professional life often turns Tanglewood into what composer and conductor Gunther Schuller, the school's artistic director, terms "the last station on the way to professionalism." Students experience professional demands in their weekly rehearsals and performances which sometimes involve 14-hour days of playing.

They see what commitment means when concertmaster Joseph Silverstein rehearses all day with the symphony and returns at night to give a solo recital in the concert hall. They comprehend the bond of affection music forms when they see a visibly tired Ozawa and Schuller come to that same concert in support of a colleague's music. They are exposed to the underside of the profession when they overhear an orchestra member complaining or watch a bored player go mechanically through the notes at a rehearsal. They measure themselves, the nature of their commitment, and, in many cases, undergo a major transformation.

"Tanglewood was the greatest experience of my life," recalled composer Sheila Silver, a student in 1972 who returned this year because the student orchestra premiered a work of hers. "At the end of that summer was the first time I said "I am a composer," rather than "I study composing," It was nothing but eating, sleeping music. As people listened to my ideas, took them seriously, I realized I was no longer just a student with nothing to say."

What the Boston Symphony gains from the students may be less obvious through no less vital. The inquiring, idealistic, energetic presence of these young people unquestionably sharpens the atmosphere.

"It would be terrible without the students. It would seem so empty," said principal second violinist Marylou Speaker, who was herself a student at the school in the '60s. "At every rehearsal they are there and I am aware of them. It makes a difference knowing that a critical audience is sitting out there."

At least once a season for a special concert titled "Tanglewood on Parade" the students and Boston Symphony members sit side by side in the shed to perform. The results, at least during rehearsals, can be unsettling for these professionals, who rank among the world's finest orchestral players and expect to perform their job with unfailing skill and ease.

"We are like dogs hitched together pulling a sled," explained Joseph Hearne, a bass player with the symphony for 17 years. "Suddenly everything comes unglued when these young players come and sit in between us because we are not in our regular places." With a laugh he recalled a principal player who came in early on a solo passage. "He had never before - never, ever that I can remember - come in at the wrong time. I don't know what happened, but fortunately it was just a rehearsal."

Though there is no way to prove it, this kind of experience probably helps keep all the orchestra's players on their toes and does wonders to counter the threat of routine which plagues every orchestra.

One of this year's most unusual works was Gunther Schuller's "Deai," which received its American premiere last weekend as part of the Fromm Festival of Contemporary Music which has been held at Tanglewood every year since 1963.

Written for the orchestra's 1978 Japan tour, the massive work utilizes the Boston Symphony onstage and the Berkshire Music Center orchestra, divided into two groups, offstage. As the work progressed, the students gradually joined the Boston Symphony members onstage until at the end all were united in what Schuller described as a "symbolic musical handshake." In Japan, the students were from Ozawa's former school.

Underlying all student and professional activities at Tanglewood is a deep commitment to the public. Audiences are welcomed as the final piece which completes the picture, for they, too, were a part of Koussevitzy's vision. He urged musicians to "be aflame with scared love for that which we serve and those whom we serve - that is to say, living art and living men."

It was Koussevitzy's genius to place performance and education side by side, said Ozawa. "Tanglewood is not simply great performances by a symphony orchestra for an audience. It is the school, this atmosphere of study. When people are getting out of the car, coming through the gate, they have expectations even before they get to the shed. That kind of atmosphere is very ideal. It means we can do some special programs and still have an audience where some other places cannot."

Walking across the lawn or in nearby towns, performers are often stopped by members of the public. "It's fabulous," said pianist Cynthia Peterson. "They'll say, "We heard you last week and you were great" or "When will you be performing again?"

Even composers are greeted. "It's the first time I've ever been stopped on the street and had people say, "I liked your work," " said 25-year-old composer Robert Carl, shaking his head in wonder.

It is the sense of participating in a tradition which engages the public. The person who stops to commend the young conductor may recall congratulating a boyish-looking Ozawa in the same hall decades before. The donor of a composing fellowship may wonder if he is supporting a future Copland.

That was the ultimate genius of Koussevitsky, to perceive that in the gently beauty of the Berkshires he was initiating a process of growth which would bond professional, sutdent and audience in a total community of affection and respect. Seldom can the past be seen so visibly shaping the present, the present so unmistakably giving birth to a future of promise as in the unfolding vision of Tanglewood. CAPTION: Picture 1, Gunther Schuller with a Tanglewood student.; Picture 2, Leonard Bernstein conducting a student orchestra during rehearsals. By Walter H. Scott