EVERY SATURDAY morning Coolidge High School in Northwest D.C. comes alive with the sounds of music. In the auditorium, conductor Lyn McLain is pulling the strings of the D.C. Youth Orchestra -- the most senior of the four orchestras that operate under the D.C. Youth Orchestra Program -- through the Brandenburg Concerto.

In the gym, the elementary orchestra is tuning up, while in three floors of classrooms, beginners are making their violins, violas, flutes, oboes, clarinets, bassoons, saxophones, trumpets, French horns, trombones and cellos sing such tunes as "Mississippi Hot Dog" and "Long, Long Ago."

"I used to play the cello bass," chimes in six-year-old Ellen Pfeiffer hurrying along to her class, "but my Daddy says you can carry the violin more places."

Some 1,700 students from ages five to 19 receive free instruction in the instrument of their choice through the youth orchestra. If they don't have their own instruments, most students can rent them from the orchestra for $50 a year. The orchestra is 25 years old this month and has produced musicians who now play with the National Symphony, the New York Philharmonic, the Boston Symphony and other orchestras. It's still directed by its founder, Lyn McLain.

"We're a program that offers an opportunity to learn to play and to perform in an ensemble," says McLain, a former music teacher at Coolidge High. "Performance is the key -- performance is the place they get to show what they've done."

Even the beginners perform regularly, in concerts for fellow students and families.

"One more time from the top," says trumpet instructor Cathy Fronck, prepping her class for a concert. "That was much better, much better than yesterday."

"Okay, that was a little better," says oboe teacher LesleNowell in a classroom down the locker-lined hall. "You have to play at least two notes before you take a new breath. That keeps the notes connected so it doesn't sound so choppy. Let's play four measures and see if we can keep the notes together."

"One, two, ready, go!" says instructor Ron Aufmann, snapping his fingers to urge on his beginning clarinet students. "Good, people. Are we ready to speed it up yet? No? Okay, we'll play it anyway."

"Ready, violins? Let's see some good playing positions. Let it ring. Let's hear nice ringing A strings. A ring means just let it sound," says teacher Kris Kuny, as his class rings out a rousing rendition of "Mississippi Hot Dog."

"Perfect!" trumpets teacher Terry Alvey to his novice trumpeters. "See, you can do it . . . Now a little faster. Yeah, that's great! If you mess up, keep going. I'm going to play it with you."

In still another classroom, eight-year-old Chris Bass is trying out a brand new bassoon.

"He just got this today," explains ior Nicholas Wilkinson. "He's just big enough to handle it. Chris, you have to lift this finger up all the way. Let me see that thumb move."

As concert hour approaches, eager and nervous young musicians stream down the stairs and through the halls to the gym, where parents and students not performing that day are assembled on bleachers. The sound of the performers tuning up mixes with the noise of babies crying. Siblings squirm, but parents are all ears, and pride.

"This is wonderful," says parent Annie Houston, as her son raises the bow to his violin. "What's fun about music is making it with other people." TUNING IN TO THE ORCHESTRA

Young people between the ages of five and 19 residing anywhere in the metropolitan area are eligible to register for the D.C. Youth Orchestra Program. Returning students, or students who already play an instrument, should register Saturday at 9 a.m. at Coolidge High School, Fifth and Tuckerman streets NW. For beginners, registration will take place on Saturday, September 14, at 9 a.m. at Coolidge.

Those who join can expect to spend two to four hours every Saturday morning in classes and practice. Members of the most skilled orchestra also practice Tuesday and Thursday evenings. Home practice is expected of all students.

The orchestra's fall/winter concert series begins at 7 p.m. October 20 at the Coolidge auditorium. Tickets, available at the door, are $1 for students, $2 for adults. Subsequent concerts will take place on November 24, January 19, February 23, April 13 and May 18. A special 25th anniversary concert will take place at the Kennedy Center on December 29. For information on joining or listening to the orchestra, call 723-1612.