The Prince George's County school system got a majority of black students this September for the first time, as the number of white students continued to decline. At the same time, the number of virtually all black schools increased from one last year to five.

The county, which is among the 15 largest school systems in the nation, joins Chicago, Philadelphia, Detroit and Baltimore in that group that have black majority systems.

The increase in the percentage of black students, from 49.9 to 52.1 percent, "was the smallest percentage change in recent years," according to Assistant School Superintendent Edward M. Felegy. But he said he doesn't know "whether it portends a particular trend or not."

School officials, currently defending in federal court charges by the NAACP of fostering school segregation, say the increase in the number of schools that have 90 percent or greater black enrollment disclosed by new statistics is a demographic phenomenon caused by drastic population shifts in the last decade.

But NAACP lawyer Thomas Adkins asserted that the new figures, including a doubling of the percentage of schools 85 or more percent black, "support our initial contention that the board has been deficient in its responsibility."

The number of white students has dropped each year since 1970, when there were 128,000, because of declining birthrates and the exodus of about 170,000 whites from the county. This year there are 50,348 white students. The number of blacks in the schools increased from 31,000 in 1970 to a peak of 60,793 last year, but this year the figure declined to 60,569. The white student decline this year was nearly 6,000.

Felegy said the jump in the county's black population from 13 to 37 percent during the 1970's tells the story of what is happening in the schools.

"If you look at the census figures, the most exact population count in the county, there was a tremendous influx of black families of childbearing age . . . that's the explanation," he said.

As for the increase in substantially all black schools, a majority of them inside the Capital Beltway and south of Central Avenue, Felegy said, "in the areas that these schools service, obviously the clientele has changed."

NAACP General Counsel Adkins maintains that the county school board is responsible for the failure of a 1973 desegregation plan and the trend toward one-race schools because they have failed to act affirmatively to prevent sharp changes from white to black in some schools.

"The white parents . . . conclude that the school board will permit segregation to occur and flee. The school board has to take responsibility for some schools becoming whiter and others becoming blacker," said Adkins, who added that he feels the new school data will not be needed as part of the re-opened school desegregation suit pending before U.S. District Court Judge Frank Kaufman.

Felegy said he expects no changes in the school system because of the arrival of a black majority.

"Students are students," he said. "The demands of parents have always been 'we want a quality program for our kids' -- I don't see that changing."