The District's public school population will rise this year for the first time in 14 years, reversing a steady decline that has concerned school officials, Superintendent Floretta McKenzie said yesterday.

The expected 1 percent increase, equal to about 700 or 800 more students, is welcome news because it will mean more stability for teachers' jobs and could prevent some city schools from closing, McKenzie said.

The District's predicted upturn after a long decline mirrors a national trend. The country's public school population peaked at slightly more than 46 million pupils in 1971 and dropped each year since to an estimated 38.925 million in 1984, according to the National Center for Education Statistics.

But the nation's school population is projected to rise next fall by about 50,000 pupils to 38.977 million, said Dr. Vance Grant of the national center.

"It's a beginning of a new wave," and is a result mainly of an upturn in births in 1977, Grant said.

The District's school population peaked in 1971, as well, school officials said. In the last decade, enrollments declined by one-third, from 132,306 in 1974 to 87,927 in 1984, according to school system figures.

"We are very pleased we are finally seeing an increase," McKenzie said.

Most Washington area jurisdictions experienced increased enrollments last fall, including Montgomery and Arlington counties, while the Prince George's County school population continued to decline but at a slower rate than expected.

McKenzie said the largest increases in school population next fall will be in upper Northwest Washington. This is a result primarily of increased births a few years ago, meaning higher enrollments in the early grades, but some of the rise is from students coming from private schools, she said.

McKenzie testified yesterday before the House Appropriations subcommittee on the District, which is holding hearings on the District's fiscal 1986 budget.

She also told the subcommittee that the school system plans to start a new system of teacher incentives next fall, including creation of a "mentor" program and paid department chairmanships.

Outstanding teachers will be designated as mentors to new teachers and will be paid extra, McKenzie said. Department chairmanships, now an informal volunteer job, will be paid, and filled with experienced teachers who will receive a $1,500-a-year bonus, she said.

Congress appropriated $635,000 last year for school and teacher incentive programs in the District.

Individual teachers and schools also will be awarded funds out of the incentive program to enable them to test plans they develop for improving teaching practices.

Bilingual programs, particularly for Spanish-speaking students, need to be expanded in the city's junior high schools and high schools, and the city plans to seek increased federal funding to do so, school officials testified yesterday. Some teen-agers coming in from Central America have never been to school at all, and this "does pose a real problem," McKenzie said.

McKenzie also said the school system plans to start an aggressive program to recruit teachers in anticipation of a nationwide teacher shortage, and that already the small number of student teachers that train in the District "is a very serious problem." The school system had only about 25 or 30 student teachers last year, she said.

William Simons, president of the Washington Teachers Union, said that some District classes are too large, with 35 to 38 pupils, and that more elementary school teachers should be hired because of the expected rise in the lower grades. Average teacher pay in the District now is about $28,000 a year, he said.

The superintendent disputed D.C. Department of Human Services statistics that showed the public school dropout rate at 32 percent. She said some of that is simply migration out of the city, and that the true dropout rate is 16.9 percent.