The Fairfax County school system finally has a uniform, comprehensive policy for closing elementary schools, but has not decided how to apply it.

The school board adopted the policy last week, after a special citizens committee and school system staff members spent a year studying the system's falling enrollment and ways to handle it. Fairfax schools have lost more than 2,000 students a year for five years, with this year's enrollment at about 129,000.

The difficult task of selecting schools to close will not begin until a year from now, when groups of schools are chosen for closing studies.

In the meantime, the school administration is conducting a massive, countywide study of student populations within present school boundaries, to determine how students can be more equally distributed in the county. School population changes in the last several years have left many schools in the older, eastern parts of Fairfax underfilled, while schools in the rapidly developing western parts of the county are overcrowded.

The problem, still unsolved by the school board, is whether to first change school attendance areas according to the results of the boundary study, or close schools according to present attendance areas, and then change boundaries.

Last year, 25 elementary school had fewer than 350 students. Under the new school closing policy, a school can be considered for closing if it has too few students to fill two classrooms for each grade - roughly 350 students.

If the boundaries remain the same, those schools probably would be examined for possible closing. If the school attendance areas are changed, using the information of the countywide boundary study, the specific schools and the number of schools to be selected for closing consideration also are likely to change.

The school board will discuss results of the countywide boundary study at a workshop tonight.

Under the school closing policy, a school also can be looked at for closing if the cost per pupil is significantly higher than the countywide average, and if it is a candidate for major renovation.

When the school board identifies schools that ought to be considered for closing, probably by December 1979, those schools will be studied along with a group of surrounding schools to ensure that a nearby facility would not be a better one to close.

A long list of criteria then will be applied to each school in the group. The criteria include the quality of education, physical condition and design, location, present enrollment and potential future enrollment, the type of community surrounding the school, the number of students who walk to classes and the impact of moving students to other schools.

The school system plans during the next few months to define the criteria more specifically and to choose a way to provide information on school closing issues to affected communities.