THEY CALL HIM "Mr. T"; he calls them "guys," and together they make beautiful music . . . most of the time. There's a special rapport between Mick Trowbridge, 33, and the teen-agers he teaches in the Prince William County school music program. As the strings instructor at Gar-Field High School, Trowbridge holds the distinction of leading the county's largest high school orchestra program. Seventy-five students play violins, violas, cellos and basses under his baton in two orchestras. And from all appearances, they love it.

How do you turn worshippers of U-2 and Pink Floyd into performers of Bach and Mozart? Trowbridge does it with a combination of dry wit, enthusiasm and guile. The students obviously adore him, and he returns the affection 98 percent of the time, he says. His recipe for student motivation is simple. "The kids take orchestra because they want to play. So I let them play."

Actually he gives them no choice. LIke a good sports coach, he drives them to play, never letting up on the tempo. At a 55-minute rehearsal recently of the Gar-Field chamber orchestra, made up of 22 students who audition for the privilege of playing in it, the pace never slowed from the time Trowbridge turned on his automatic tuner for an A until the students packed away their instruments and headed to their next class.

"Watch your flats," he called out once over the whining sound of violins going sharp. "B-FLAT!! . . . What? Your violin not cooperating?" Tapping relentlessly on his music stand and mouthing a constant stream of ba-bum-ba-bums and da-da da-das, Trowbridge put the group through its paces with an A-Flat scale, Beethoven, Holst, Mozart, Bach.

"Sounds pretty bad, guys," he said at one point. "Keep going though." And go the guys did, feet tapping on the linoleum, fingers flying along strings. "Good!" he said, brightening. Then, "Aw, come-on. Make it sound better," a few minutes later.

The hour was exhausting, exhilarating, occasionally frustrating, and all organized toward a purpose, the making of music -- something Trowbridge has devoted his adult life to. A graduate of James Madison University, he plays every stringed instrument in the orchestra, but his specialty is the bass. He began teaching in 1976 in Prince William County and married another music teacher. Her middle school program feeds into his.

But Mr. T doesn't work only with the relatively accomplished musicians in high school. Every day he goes to Enterprise and Dale City elementary schools, both so overcrowded in this burgeoning county that he teaches in a trailer in Enterprise and the cafeteria at Dale City, where the dishwasher goes "ga-thump, ga-thump" to the beat of the music. It's all in a days work for a school music teacher.