Fairfax County's school enrollment, which has been declinging steadily for the past several years, fell again this autumn as the schools reported the loss of another 2,788 students.

The annual student count, taken in September, showed Fairfax's total school enrollment at 129,524, compared to 132,312 last year. In September 1975, enrollment was 136,944.

Declining enrollment has caused a myriad of problems for the school system. These include the closing of schools, providing a full schedule of classes at schools with too few students to fill each classroom and difficulty in convincing voters to support bonds to build new schools in the western parts of Fairfax where enrollment are still climbing.

The Fairfax School board is sheduled tonight to adopt a new, comprehensive school closing policy that was developed after a year of work by local citizens and school planners.

The policy will set guidelines for choosing a group of schools to study for closing and will set criteria for selecting which schools should close.

Several schools, particularly in the older, eastern areas of the county, are expected to close within the next 10 years. If the board adopts the policy, it will mark the first time Fairfax has had a single uniform policy under which to close schools. Fairfax so far has closed only five schools due to declining enrollments.

At the same time, the board is conducting a massive countywide boundary study in response not only to falling enrollments, but also to the need for a broad overview of where students are assigned to schools.

The computerized study ignores all previous school attendance areas and looks at theoreatical options for how students could be assigned. One computer study, for example, examined the effect of assigning all students to schools closest to their homes. Other studies examined where students would be sent if a bus ride lasted no longer than 40 minutes, and if a school were filled to only 85 percent of capacity.

The school board will discuss the results at a public workshop next Thursday.

School officials are calling the study a "decision-making tool," saying they do not intend to shift a large number of students at one time if the results of the study show that school could be better used through such a move.

"Some of our worst troubles have come over deciding to close just one school," said Rodney Page, chairman of the Fairfax County School Board. "There's no way we could just ignore established boundaries in making decisions."

The results of the study will be used to help the school board make decisions regarding its annual school boundary changes, and in closing schools. They also may influence how many bond referendums are held to finance building new schools, Page said.

"Theoretically, the board could defer building a new school, if the results of the study showed students could be accommodated as well in existing schools," Page said.