They seem so precocious, these teen-agers, sitting behind the desks at "It's Academic," poised to push a button almost as soon as the question is asked -- and sometimes before it's even completed.

But the man who poses the questions, Mac McGarry, has seen a whole generation of bright young high school students grow up, and he advises adults suddenly struck with self-doubt that while "it's wonderful to see a display of knowledge, I tell people not to be intimidated . . . For one thing, there is more to be known now. Think of what had not happened in the early days."

The questions they get aren't harder, he reassures. There's just more to ask.

McGarry should know. He's been hosting the popular quiz show for 24 years at NBC's Washington affiliate, WRC-TV (Channel 4), and for 12 years at the Baltimore affiliate, WBAL-TV (Channel 11).

Sunday he'll host the Washington- area finals (9 a.m. on 4), and Saturday he'll preside over the 14th annual "It's Academic Super Bowl" (7 on 4 and 11) between the three-man winning teams from those two regions, Timonium's Dulaney High School -- last year's Super Bowl winner -- and Bethesda's Walt Whitman High School, which won the first intercity competition in 1972.

The Dulaney team will bring along the traveling trophy, newly shined and ready for a 1985 engraving. The team they beat last year, Washington-area champs Rockville High School, will return the second-place trophy for reassignment.

This year, producer Sophie B. Altman has arranged for a taped introduction by President Reagan, who says that " 'It's Academic' offers a positive image of young people" and commends the show as "a scholastic battle of wits, a showcase for bright young students and a reminder of the importance of education."

Off the air, McGarry notes that "this is a show that requires constant attention," and compliments Altman and her staff, who write the questions (never one used twice), rotate the schools and coddle the studio audience as well as the teams. McGarry is also full of praise for Giant Food, Inc., which has backed the show since its inception. "You couldn't ask for a better sponsor -- they seem to appreciate what we're doing," he says. During the year, Giant awards scholarships to participating schools (the two winning teams, having worked their way through four matches, earned $2,350 each) and pays for the production of the shows both at WRC and WBAL. Al Dobbin, a senior vice president, will award the trophies at the end of Saturday's competition.

In turn, Altman credits the enthusiastic McGarry with being "very smooth . . . knowledgable, witty, urbane and articulate."

McGarry was just one of several NBC staff announcers when he was assigned to work with Altman and her fledgling program 24 years ago, when it was just an experiment. "And like a good, Jesuit-trained boy, I did what I was told," he said.

Though he is still an announcer with the NBC affiliate, doing commercials and public service announcements and voice-overs, he also has a radio program of his own from noon to 4 p.m. Saturdays where he plays his favorite big band music ("I'm a fan of Peggy Lee") over WRC-AM. And he devotes a chunk of his time to taping "It's Academic" -- 40 shows here and 36 in Baltimore, plus the annual year's end "Super Bowl" edition.

He's on the lecture circuit, too, often speaking at high schools and hosting spin-offs of "It's Academic." He recently presided over the District's school-system sanctioned 11th-grade finals held at Theodore Roosevelt High School.

McGarry says he is proud that "a local television program can make a contribution," and points out that "'It's Academic' is now a recognized extracurricular activity in every jurisdiction (in the Washington area). There are 'It's Academic' clubs within the schools . . . (and) among the schools."

And he is pleased, too, that on his show, "academics is getting the attention that athletics gets." Some schools, particularly those with a history of intense practice for the competition, give letters for participation in the "It's Academic" program.

Each month, McGarry and Altman and crew tape four local competitions in one day, filling the studio with students, parents, teachers and friends, high school pep bands, pompon girls and cheerleaders.

Unlike the format of the two-team "Super Bowl" edition, the school-year "It's Academic" pits three teams of three students each against each other and against the clock. Quizmaster McGarry keeps the competition moving rapidly as he reads the questions from small cards and rules decisively as teams win points or lose them for wrong answers.

McGarry, who conveys the bearing of a bespectacled professor giving a richly varied and very difficult pop quiz, maintains that he would love to "let go" and entertain the students and their audience ("I'm a riot," he claims). But he doesn't. "I'm constrained by time -- if I did, it would come out of the kids' time."

The world of teen-agers is not unfamiliar to Mac McGarry. He and his wife, Babette, who live in Potomac, have four children of their own -- Steve (like his father and grandfather, also called "Mac"), 25; Laura, 24; and 18-year-old twins, Mark, who attends Montgomery College, and Andrea, a student at McGarry's alma mater, Fordham University.

McGarry, born in Atlanta, grew up in New York City and attended Regis High School and Fordham, where he majored in communications ("we were pioneers -- we had our own FM station.") He recalls listening to "Quiz Kids" on the radio and watching "College Bowl" on television, and says his mother, perhaps foreseeing his future, made certain tha he had neither a southern nor a New York accent. After a short stint at a station in Pittsfield, Mass., McGarry joined NBC here in 1950.

As host of the show, McGarry has seen an entire generation of adolescent scholars compete on "It's Academic," and observes that over the years "they have been a reflection of the way our country was." Today, he says, seems to be "a calmer time" than in the late '60s and '70s, when one student, asked his plans for the future, responded that he intended to smoke a lot of marijuana.

In the spring of 1981, McGarry presided over a reunion of about 400 of the quiz whizzes in the Cannon House Office building. The most popular profession -- 16 percent of the former team members -- turned out to be law. One man had become a shepherd. And there was at least one romance: Two former competitors from opposing schools enrolled at Princeton and later married.

On Sept. 22, when "It's Academic" begins its silver anniversary year, Altman says Maryland Gov. Harry Hughes, Virginia Gov. Charles Robb and District Mayor Marion Barry are scheduled to proclaim an official "It's Academic Day."