The number of students in Washington's public schools dropped his fall by nearly 6,000 - the steepest decline yet in a decade of tumbling enrollments, according to new figures released by the D.C. school system.

The official enrollment count this fall, taken Oct. 20, was 119,965, compared to 125,903 a year earlier - a decline of 4.7 per cent.

Since 1969, when enrollment in District schools hit a peak of 149,116, the number of students attending schools here has dropped by about 29,000 or 19.5 per cent.

Officials said the decline is part of a nationwide trend caused by a downturn in births, which has also affected schools in the Maryland and Virginia suburbs. However, the enrollment drop in Wahington schools started earlier and continues to be steeper than elsewhere in the area, reflecting the city's continued loss of population to its suburbs.

The decline this year also is about 2,500 students more than D.C. school officials anticipated when they drew up the budget for the current school year. As a result, school Supt. Vincent Reed said average class size now is about the same as a year ago instead of rising slightontinuing just as the city government is completing its largest school construction program ever. Some 20 new schools and additions have opened in the past three years.

Since very few old schools have been closed, many of the new ones are considerably under capacity.

For example, the Friendship School, a combination elementary and Junior high at South Capitol Street and Livingston Road SE a capacity of 2742 students, but only 1,753 enrolled this fall.

The enrollment at Fletcher-Johnson, another large new Anacostia school, is 49 per cent of capacity.

Reed said yesterday he will present a proposal to the school beard in January to close some schools next year, but he decloned to say how many buildings or which schools would be involved.

According to the new report, the enrollment decline is taking place in all parts of the city. Even the area east of the Anacostia River, whose school population had mushroomed until the early 1970s, lost 4.3 per cent of its public school students last year.

Although the new report contains no figures by race, it indicates that the enrollment drop was relatively slight in the city's only small pocket of mostly white elementary schools, west of Rock Creek Park. The decline there this fall was 3 per cent, the report said, compared to 6.5 per cent for all elementary schools in the city.

In 1976, the D.C. school system was 95 per cent black, a proportion that has been stable since 1970.

Among the few elementary schools that gained students this fall are Eaton Elementary School in Cleveland Park - up 9 per cent to 348 students and the Edmonds-Peabody School on Capitol Hill - up 25 students to 324.

The school with the largest enrollment increase, without boundary changes, was the Stevens Elementary School, at 21st and K Streets NW, which grew from 209 to 251 students. The school is attended by President Carter's daughter Amy and has an after-school day-care program that has attracted students whose parents work nearby but live all over the city.

Even though the bulk of the enrollment decline continued to be in elementary schools, the city's junior high schools lost 1,509 students compared to last year, or 5.2 per cent. Senior high and vocational schools, which previously had been gaining students, had their first slight loss - down 162 students from a year earlier.

"It looks like the decline over the past few years in the elementary schools is finally hitting the secondary llevel as well," Reed said yesterday.

Many junior and senior high school still have more students than their rated capacities even though some junior highs, notably Gordon and Lincoln in Northwest, are markedly under-enrolled. Reed said none of the over-capacity problems is serious enough to warrant transferring students to under-used schools.

"You've got to talk about the location of schools when you talk about how full they are," Reed said. "I don't want to create a busing situation."

In contrast to the general enrollment decline, the report shows an increase of 146 in the number of children attending full-time special classes for the handicapped - a 10 per cent rise in one year to 1,533. The school system is under a court order to expand its special education programs.

Overall, the 119,965 students attending Washington public schools this fall constitute the smallest enrollment since 1959.