The District school system, which spent $300 million on new construction in the last 15 years, now has almost a third of its seats empty as enrollment has continued to fall and efforts to close underfilled buildings have been stymied.

According to a compilation of data from the D.C. Board of Education, at least 50 percent of all seats are empty at 30 of the city's 149 elementary and junior high schools. In 41 elementary and junior high schools, 40 percent to 49 percent of the seats are unoccupied.

Overall, the school system has almost 42,000 excess seats. There are 87,000 students enrolled. Officials say that despite their cost, many underfilled buildings are justified because they operate far more comfortably than they would if fully used, particularly the massive buildings in Northeast and Southeast Washington that have large open-space classrooms.

Last year, the school system spent $67 million -- about 18 percent of its operating budget -- on cleaning, heating, and maintaining 198 buildings. But maintenance has lagged so badly that, faced with widespread complaints about shabby conditions, the City Council added $8 million for school repairs last week to next year's budget.

The last efforts to close large numbers of schools with low enrollment rates were made by former superintendent Vincent Reed in 1978 and by Superintendent Floretta D. McKenzie in 1982. Almost all their recommendations were rejected by the school board because of community opposition, although since 1978 the number of elementary and junior high schools operating here has gradually been reduced by eight.

The city's 13 regular senior high schools are filled to more than 90 percent of rated capacity. But in 1983 the number of available seats shown in school system reports was cut by about 3,000 when five vocational high schools were converted to part-time career centers.

Vocational students now spend about half their time there and half taking academic courses in the regular high schools. They are counted in the reports as enrolled at the senior highs with no adjustment for part-day use of the buildings.

To fill the unused space, school administrative offices have been moved into empty classrooms in many buildings, and space in 26 schools has been rented to outside organizations.

Last year, the school system collected $447,000 in rentals, including fees from occasional users. But the school board has no central inventory of how its space is being used, although a listing is now being prepared.

"D.C. public school space is notably cheaper than any commercial space in the city," Daniel Wright, the system's leasing officer told the board's buildings and grounds committee on Tuesday. According to figures provided by Wright, the rental charges average $3.60 per square foot a year.

"I think we have quite a few squatters, too," said Wanda Washburn, the committee chairwoman and school board member who represents Ward 3. "The principals often don't tell us what's going on because they are afraid we will tell them they can't do it any more. A lot of these uses are worthwhile, but it behooves us to know about them."

Washburn and Eugene Kinlow, an at-large school board member and a former buildings and grounds committee chairman, said much of the apparent underuse is justified because new programs, such as computer labs and remedial instruction in reading and math, use the space.

"There's a different philosophy of delivering educational services," Kinlow said, "and that causes us to utilize more square feet per pupil" than when building capacities were calculated by architects and budget planners.

However, a few D.C. schools with well-regarded programs are substantially over capacity, often attracting many students from outside their official attendance boundaries. For example, Oyster Elementary, a bilingual school at 29th and Calvert streets NW, is 32 percent over capacity. Many of its pupils come from the Adams-Morgan area, where the Reed School, 18th and Champlain streets NW, has one of the lowest enrollment rates in the city, operating 70 percent below capacity.

Jefferson Junior High, Eighth and H streets SW, is the city's most crowded, according to the enrollment report, at 52 percent over capacity. Its enrollment of 799 is up by almost 250 since it started building a strong academic program in the late 1970s. The school has been nationally recognized and visited by President Reagan.

"That's what happens," Kinlow said, "when you have a good reputation and some kind of open enrollment."

According to the enrollment reports, the largest clusters of half-empty buildings are in Ward 6 around Capitol Hill and in Ward 7, around Deanwood and in neighborhoods near East Capitol Street, east of the Anacostia River.

On Capitol Hill, enrollments have plummeted over the last decade as black families have left because of gentrification, but the area is one of the few in the city where a substantial number of small old schools have been closed. In Ward 7, a major building program executed in the 1970s, at the same time population was declining, turned an area with crowded schools into one with surplus space.

Ward 3, west of Rock Creek Park, has the city's highest school occupancy rate. It also has some of the city's smallest schools and, according to Washburn, many students come from elsewhere in the city.

Since its peak enrollment of 149,116 in 1969, the D.C. school population has dropped by 42 percent. Officials project a slight increase next fall -- the first since 1969 -- but the upturn over the next few years is expected to be far less than the previous decline.