Paul Johnson, an 18-year-old junior at McKinley High School, stood up in the first row of the crowded City Council chambers, looked straight at Mayor Marion Barry and asked his question with all the confidence of a veteran reporter.

"What are your future plans if you don't win the election?"

Barry, who grinned at the question, drew laughter and applause when he replied: "With all the help I am going to get from people in D.C., Maryland and Virginia, we are going to win the election."

He added that he "probably would go into business" if he lost the Sept. 14 Democratic primary, saying that he was not worried about being unemployed because he had "marketable skills."

Johnson was one of the 31 high school students from the District and more than 90 others from Maryland and Northern Virginia who met the mayor last week at a "press conference" arranged by the Close Up Foundation, which holds week-long seminars in Washington for students from around the country. Last week's seminar was for local students.

Clearly enjoying his encounter with the students, Barry deftly fielded questions ranging from politics to the city's policy on sheltering the homeless. He also managed to do a little campaigning.

Another political question came from Robin Kinard, 16, a junior at Roosevelt High School and the Penn Career Development Center, who wanted to know if the mayor was "surprised" with the results of a Feb. 28 poll by WRC-TV News and the Associated Press in which a majority of the 439 persons interviewed said they were opposed to Barry's reelection.

Barry said that he didn't believe that the poll counted for very much.

"The poll I believe in is the final poll: the election," Barry said. "That's the poll I want to see. I predict a lot of hard work no matter who runs. But I am going to get far more votes than anyone else."

Earlier, when Aquinas Butler, 16, a sophomore at Dunbar High School, had asked Barry why he was cutting the summer youth job program this year, the mayor seized the opportunity to "advise all of you" of the importance of getting correct information.

He said that, instead of decreasing summer youth jobs, the city would increase them from 17,000 last summer to 20,000 this summer.

"Where did you get your information?" Barry asked Butler, saying that he was not trying to be hard on her. Butler, who said she plans a career in law enforcement, replied vaguely that "someone" had told her the summer youth job program was being reduced.

Barry urged any D.C. student who had not registered for the program to do so before the May 1 cutoff date because job applications are taken on a first-come, first-served basis.

During his opening remarks to the group, Barry also urged students who were 18 years old to register to vote and to "participate actively in the community."

Acknowledging that some people think such participation "doesn't make any difference," Barry said that "every major decision made in America that affects you was made by a politician."

To illustrate his point, he told the students that even "how much you pay for bread," as well as the cost of shoes, automobiles and clothes, all are determined by political officials who set wheat price support subsidies and import quotas.

The mayor also talked about his difficulties as manager of "a major urban city," saying he faced such problems as "shrinking revenues, growing demands and higher expectations."

He referred to the Reagan administration's economic policy as the "New Meanness" and said cities and counties around the country are "suffering" because the federal government "has cut back a great deal of its commitment, saying help yourselves."

After the meeting ended, Carmella Saunders, a 17-year-old junior at Roosevelt High School, said she did not get to ask the mayor her question: What could he do about the overpopulation of D.C. schools?

But Saunders said she still thinks Barry is "a good mayor. He can't change everything all at once," she added.

Other Roosevelt students, senior Edimae Whyte, 18, and juniors Saundra Fowler, 17, and Bonnie Scott, 16, agreed.

Whyte said she thinks the mayor is "just excellent. He was pretty good today, too," she added. "It was very interesting."

Fowler and Scott both said they think the mayor should be reelected.

One dissenter, Aquinas Butler, disagreed with the Roosevelt students but declined to elaborate, saying only that she didn't like the mayor.

Roosevelt High School, with six students, had the most D.C. representatives at the seminar. Dunbar, Penn Center and St. Cecilia's high schools each sent five students, Immaculate Conception Academy sent four students and Spingarn, Ballou and McKinley high schools sent two students.

A spokesman for the Close Up Foundation said the organization expects to sponsor "about 14,000 students" this year. Tuition for the seminars, including hotels and food, is $300 for both local and out-of-town students.