The number of black students in Washington's public schools fell this fall by 5 per cent - its steepest drop ever - while the small number of whites in the city school system increased slightly.

According to a new enrollment report, there are 113,813 black children in the city schools this fall, down 5,999 or 5 per cent from a year ago.

Meanwhile, the number of whites in public schools is 4,422, which is 16 children more than last year. In addition, the school system has 1,062 Hispanic students, 635 Asians, and 33 American Indians.

Overall, blacks now make up 94.9 per cent of the District's public school students - the highest proportion of blacks in any big city school system in the country. However, the proportion is down slightly from its high of 95.7 per cent black, reached in 1973.

That year also marked the low-point in the city's nonblack enrollment - 5,812. This fall the total number of nonblacks is 6,152.

Officials said the decline in the District's black school enrollment is part of a nationwide trend caused by a down-turn in births, and also reflects the continuing migration of blacks from the District to its suburbs. At the same time, migration of blacks to Washington from the South has virtually stopped.

On the other hand, the exodus of whites from the city's schools, which proceeded rapidly for almost two decades, apparently has stopped, although no sizable return has taken place.

Last month a new official estimate of the city's population for 1976 showed the first increase in the white population here in a quarter of a century.

Statisticians said the increase of about 6,400 whites brought the proportion of whites in the city's population up to 23.9 per cent. They said most of the increase has taken place in older, in-town neighborhoods such as Adams-Morgan, Capitol Hill, Logan Circle, and Shaw, where extensive renovation of old houses is taking place.

However, the new school enrollment report indicates that few of the white newcomers have children in the public schools. On Capitol Hill, the Edmonds-Peabody Elementary School showed a slight increase in its white enrollment to 35 per cent, but the number of whites in another Capitol Hill school. Brent Elementary, fell slightly to 30 per cent.

There are still virtually no whites in the public schools in other neighborhoods where renovation is occuring.

The enrollment report indicates that white students are still concentrated west of Rock Creek Park, the only large area of the city that is predominantly white. Eight elementary schools there have white majorities, including one, Mann in Spring Valley, which is over 90 per cent white.

None of the city's 48 junior and senior high schools has a white majority. However, at Deal Junior High, near Tenley Circle NW. the number of whites increased to 39 per cent of enrollment. At nearby Wilson Seniro High, however, the number of white dropped to 20 per cent of the student body.

Two elementary schools in Anacostia - Leckie and McGogney - reported an increase in whites to about 25 per cent of enrollment. Most of the white children come from Bolling Air Force Base.