D.C. Schools Superintendent Floretta McKenzie, struggling to pare down a system with space for 40,000 more students than it now serves, yesterday expanded her list of proposed school closings to include an award-winning facility in Ward 3.

McKenzie was criticized by some board members earlier this week for including no schools from the mostly white, affluent Ward 3 area in her initial list of 12 buildings to be closed. At-large member Eugene Kinlow requested that she give the board some additional schools to choose from, and in response she yesterday proposed the Fillmore Arts Center in Georgetown and the Old Congress Heights school in Anacostia.

The board members then put off action on the closings until April 26, saying they needed more information on all 14 schools before making any decisions. Some members said they had found inaccuracies in the study on which McKenzie's initial recommendations were based, and they asked for clarification on some points.

At the same time the board members, seeking an alternative to closing schools, voted to explore the possibility of leasing empty school space.

The board will advertise over the next two weeks the availability of space in the 14 school buildings for "governmental and social service agencies, community groups and nonprofit organizations." Several board members have suggested recently that school closings could be avoided if the system found agencies interested in renting the space.

Ward 1 representative Frank Smith cast the sole dissenting vote among the 11 board members, arguing that to postpone action "just prolongs the confusion" for the public over school closings. "We are wasting money that could be used to teach children" by operating underused schools, Smith said.

The problem of expensive, empty seats in the school system results from a massive building program in the late '60s and early '70s that coincided with a massive drop in student enrollment.

Today, Washington's public schools have space for almost 40,000 more students than are actually enrolled, a surplus that will be only slightly cut even if the board eventually votes to close every school on the superintendent's list.

If all 14 of those schools close, the system will have eliminated less than 6,000 seats. That decrease would be offset largely by the drop in enrollment of 3,700 students projected for next year.

"This is a pretty cautious plan to close some small schools that are near to some others," said one school staff member, who asked that his name not be used. "But people have such an emotional involvement with a particular school that it's hard to close anything. It's the same thing all over the country."

Indeed, since birthrates fell sharply around 1970, the agony of closing schools has confronted communities in most parts of the nation. Almost all the Washington suburbs have gone through the turmoil, too, most recently Montgomery County, whose school board voted last fall to close 28 schools by 1984.

In Washington, though, the surplus of school seats has grown to unusual proportions as the decline in enrollment has been unusually sharp and new school buildings have been unusually large and slow to reach completion.

Though the mayor and some members of Congress who oversee the city budget have encouraged school closings, school officials have been reluctant in the past to take the political heat of closing small, old neighborhood schools that parents want to keep open.

The only major effort to shut schools was made by former superintendent Vincent E. Reed in 1978. But his plan encountered fierce opposition in a month of long, emotional hearings.

When the school board voted, it sided with the community protestors in every case but one, closing just nine of the 23 schools Reed recommended. Most of those closed already held administrative offices or city-wide programs rather than neighborhood schools.

Afterwards, Reed remarked, "I'll just have to live with it. You know, some of these schools should have been closed long before now. But we'll just have to keep them going."

Since Reed's failed attempt at large-scale closings, the number of surplus seats has risen from 14,244 to 39,842. Of the 14 schools on McKenzie's new closing list, eight are survivors from Reed's list four years ago. In every case enrollment has shrunk still more.

Fillmore Arts Center is one of those eight. It currently has no student enrollment of its own, but provides a variety of weekly art and music courses for 888 students in five other Ward 3 schools. Fillmore recently received a $10,000 award from the Rockefeller Brothers Foundation for its innovative curriculum and record of student achievement.

Old Congress Heights, the other school added to McKenzie's list yesterday, houses a 39-student, federally funded preschool program and some community functions. The board voted to close Old Congress Heights in 1980, but the school system has kept it open.

McKenzie denied yesterday that she had put Fillmore on the list to counteract complaints from some board members and citizens that she had given preferential treatment to Ward 3.

"Upon reflection, in light of the fact that [Fillmore] has no constant student enrollment, I had to present it to the board" for possible closing, McKenzie said.

At yesterday's meeting, Ward 3 representative Wanda Washburn defended the Fillmore Arts Center, citing the national recognition it has received. "Good heavens, if we close this school down, we're saying we don't care," Washburn told the audience of about 20 people, including some Ward 3 parents and Fillmore's principal.

During the past 10 years, the number of public school students in Washington fell by one-third to 94,425, including a 21 percent drop since 1977, as movement to the suburbs and private schools compounded the drop in births.

During the same period, the city completed the largest school building program in its history, with 43 new schools and major additions opening since 1972 at a total cost of $252 million.

The construction program was set in motion by an enrollment spurt in the 1960s that forced the city to install more than 350 prefabricated classrooms to house students. But the new schools were built so slowly that by the time many of the new buildings were ready, there were no longer enough students to fill them.

One reason for the slowness was the convoluted budget process, in which both Congress and the city government had to approve funds. Another factor was the difficulty of displacing residents from possible construction sites, a problem that led to a policy of building exceptionally large schools on any site available.

But the size of the new schools--about 1,100 seats or more for each of half a dozen elementary schools, 2,730 for one combination elementary/junior high--created serious problems of its own.

Not only does the size itself make it more difficult to maintain discipline, but all the new schools have open-space classrooms, planned to house 100 to 150 students in a single room. Teachers complain that the large rooms often are noisy and distracting. With the new schools underenrolled, they spread out groups of students, using empty floor space to separate groups since they don't have walls.

In 1978 many parents objected to moving their children from small buildings to the large ones. Teachers and principals in the large open-space schools complained that enrolling more students to bring them closer to their capacity might overwhelm them.

The amount of surplus seats in the new schools is considerable. At Friendship Elementary and Junior High, South Capitol Street and Livingston Road SE, there are 1,591 students in space for 2,730. M.C. Terrell Elementary, at Wheeler Road and Savannah Street SE, has 523 in space for 1,124.

The most extreme example of an underused new school is Marie Reed Elementary, 2200 Champlain St. NW, which opened in 1977 with a capacity of 1,119. It now enrolls 323 students, an occupancy rate of just 29 percent, and this year has turned over some of its rooms to an architecture program for Florida A&M University.

Although in 1978 virtually all of Washington's surplus seats were in elementary schools, many are now in junior and senior highs. Enrollment drops have reached those grades, too, at the same time as new additions and modernization projects have been completed.

Also, the empty seats have spread to all parts of the city. Somewhat paradoxically, the highest occupancy rate is in the section with fewest students, Ward 3, west of Rock Creek Park, the city's only heavily white area. Its schools are 83 percent filled, compared to 82 percent in Ward 8 in Anacostia and between 72 and 64 percent in the rest of the city.

The reason is that Ward 3 has relatively few schools, most of them small and spread far apart. One of its largest, Gordon Junior High, was closed with little controversy four years ago after virtually all of its neighborhood children had left.

By contrast, in other parts of the city many schools are close together. This is partly because of dense population but in many cases it is a legacy of the days of segregated schools, when separate buildings for blacks and whites were often built near each other.

In Anacostia, some schools are close together because when new ones were completed in late 1960s the old ones were still needed to relieve overcrowding and weren't torn down.