Fairfax County schools have lost nearly 2,500 pupils since September of last year, a count that confirms recent trends showing a decline in the county's pupil population, particularly at the lower grade levels.

The annual September tally of Fairfax County pupils shows that although pupil population has decreased countywide since 1975, it continues to increase in newer, developing portions of the county.

Large decreases in students at pre-school that even fewer pupils will be enrolled in upper grade levels in the near future.

These numbers do a lot more than show student enrollment," said Donald R. Schudel of the schools' planning services. "They reflect changes in family structure, in the work force and in migration trends in the county."

There were 132,312 pupils enrolled by Sept. 30 in county schools, compared to 134,781 pupils enrolled by the same time last year, according to school figures presented to the board last week.

These figures compare to 136,944 pupils in September 1975, 136,508 pupils in 1974 and 135,839 pupils in 1973.

"Our school population peaked in 1975 and now seems to be on the decline," said a school spokesman. "It would be declining faster if it weren't for fast growth in certain areas of the county like Burke Center, Chantilly, Reston and Herndon."

The greatest decline has been in the eastern portions of the county that border Alexandria and Arlington County. Schools serving Annandale, Springfield, Seven Corners, Lincolnia and their surrounding areas lost 1,520 students during this past school year. Schools serving the areas around Franconis, Mount Vernon and Ft. Hunt lost 1,367 pupils.

Fewer families are moving to these older, more developed portions of the county, which form administrative areas 1 and 2 in the county school system.

The schools' administrative area 3, which covers the western and the entire northern portion of the county from the towns of Reston and Herndon across to McLean and Langley, has seen both growth and decline.

This area lost only 115 pupils during the year, resulting from a drop in students in the eastern part, combined with greater development and more families settling in the western part.

Kindergarten lost nearly 10 per cent of its membership since September 1976, when 7,7551 pupils were enrolled. Pre-school programs lost more than 11 per cent of their membership since last year, when 181 pupils were enrolled.

Meanwhile, enrollments increased in special education and Head Start programs during the past year.

Results of the state-required triennial school census also were presented to the board last Thursday. They showed that the school system's black population has increased from 2.8 per cent in 1971 to 43 per cent. Other ethnic groups, including Hispanic and Asian, have climbed to 4.1 per cent of the school population, compared to less than 1 per cent in 1971.