For about 2,000 students at McLean's Langley Senior High School yesterday was hardly a typical day.

In the band room, Dr. E. James Lieberman, co-author of a sex and birth control guide, was giving a talk on birth control complete with a display of a diaphragm and other contraceptive devices. He gave a careful explanation of how each works.

Just down the hall, Linton Deck, Fairfax county's school superintendent, was describing how the school system operates. In response to a question, Decks said he favors busing for school desegregation.

In another wing of the sprawling high school, Senator William S. Cohen (R-Maine) blasted the Democrats for allowing taxes to rise so high, he said, that they are "killing incentives to work."

With 17 National Merit semi-finalists and average SAT scores far above the national norms, Langley is one of the top-ranked academic high schools in the Washington area. The parents of its students include a galaxy of Washington notables -- about 20 members of Congress, including Sen. Cohen, one Supreme Court justice, John Paul Stevens, President Carter's national security adviser Zbigniew Brezinski, and scores of high-ranking officials from the Central Intelligence Agency, the Pentagon and other government agencies.

Yesterday about 40 of the parents and about 40 other experts in different fields replaced Langley's teachers for most of the day in a program called SOAR -- Seminars on American Reality.

The seminar-givers included three U.S. senators, three congressmen, two CIA officials, and the chairman of the National Transportation Safety Board. But the one who drew the largest audience -- about 650 students -- was Jim Elliott, a disc jockey on radio station WPGC who appeared on the auditorium stage wearing tight blue jeans and a black and red jacket.

The slice of American reality he talked about was pop music and the radio stations that play it. And Elliott was frank.

"It's all business, unfortunately," he declared. "A lot of the entertainment has gone out of it."

In some of the other sessions, speakers talked openly about things that usually aren't discussed in school classes at all.

Gary H. Totman, an undertaker, answered questions in detail about his profession from six classes of about 30 students each.

Among the questions were:

How do you cremate people? Where do survivors put the ashes?How do you embalm a body? How do you remove an organ of someone who wants to donate it?

Totman answered them all matter-of-factly. When one blond girl asked if he needs any summer help, Totman said he doesn't.

The day's classes on sex education included not only presentations by Dr. Lieberman, a Washington phychiatrist, but also discussions led by Mary Lee Tatum, a Langley parent and a sex education teacher at George Mason High School Falls Church.

In Tatum's sessions, one girl asked: "How do you confront your parents with different subjects, like contraceptives or like you're pregnant?"

Tatum explained at length that it depends on "the relationship between you and your parents."

In his class, Lieberman was full of practical advice, such as that condoms should not come into contact with sharp fingernails or Vaseline.

"I do more talks like this with PTAs than with kids," he said later, "but it's nice to talk right to the kids. The teachers could have their jobs on the line if they talked about this. What could they do to me? Just not invite me back next year."

This was the sixth year that Langley has run its special seminar program, and the second in a row that Dr. Lieberman took part. Besides his two sessions on "sexual responsibility," he conducted two others on "Esperanto, the international language."

School Superintendent Deck said he decided to participate as soon as he was invited in mid-January, shortly after coming to Fairfax from Orlando, Fla. His class attracted only 10 students. None of them knew how many people serve on the Fairfax School Board or could name its non-voting student member (Jim Taylor). But they asked Deck some pointed questions, and he responded with strong answers.

Court-ordered busing for school desegregation, Deck said, "has been more beneficial than harmful." Even though critics say it adds to school costs, Deck decleared, "That isn't so."

"It simply means that you have different patterns of busing," Deck continued. "For many years people were riding buses, often beyond the schools closest to their homes. The problem was that those folks were black. Now with court orders it's much more equitable."