Fairfax County schools will begin school studies in December 1979 under a policy adopted Thursday night. An incorrect date was reported in some editions of yesterday's paper.

The Fairfax County School Board last night adopted a comprehensive new policy to govern closing several elementary schools in the next few years, starting in December 1979, due to falling students enrollment.

The policy, the result of a year's work by citizens and school staff, calls for studying the possibility of closing a school when it has too few students to fill two classer per grade (about 350 students), has a cost per pupil significantly higher than the county-wide average, or when it is a candidate for major renovation.

Fairfax schools have been losing about 2,000 pupils a year for five years, with present enrollment at about 129,000. Last year, 25 elementary schools had fewer than 350 students.In four years, about 60 of the county's 122 elementary schools are expected to have fewer than 350.

A school proposed for closing would be studied along with a group of surrounding schools that also could be closed. The policy sets guidelines for selecting schools to close and for exempting certain schools - such as new or newly renovated ones - at the board's discretion.

Closing studies, to be conducted over several months, would not begin until next September.