The standardized test scores of black students in Fairfax County schools trailed those of white students by as much as 36 percentile points in the 1984-85 school year, rising only slightly during the first year of a special program to improve minority achievement in the county, school officials said yesterday.

"Minority students are not doing as well as we think they ought to be doing," Robert R. Spillane, superintendent of the Washington area's largest school system, said.

Spillane and other school officials said they had not expected big gains only a year after a program to improve blacks' scores began. "There's no simple, fast solution," Spillane said. But he said the small increase in the blacks' most recent test scores compared with the previous year -- generally two or three percentile points -- is encouraging and may be proof that the county's effort to improve minority scores is paying off.

"While it's a long haul to improve them to the point that we'll all be happy, we're on the way," Spillane said.

The release of Fairfax minority scores on the standardized Science Research Associates test came two weeks after the release of figures from Alexandria that showed blacks lag behind whites by as much as 48 percentile points.

In Prince George's County, recent scores on the similar California Achievement Test showed a 20-point gap between blacks and whites.

The scores are particularly embarrassing for school officials in a county that is one of the nation's richest, and uses the academic excellence and high test scores in its school system as a drawing card for businesses and residents.

The Fairfax scores released yesterday showed most white students scored about the 80th percentile in ability and achievement, while most blacks scored around the 50th percentile, the national midpoint. Black scores trailed those of whites by 36 percentile points in second grade mathematics and educational ability.

Although scores of whites have remained relatively steady since 1983, those of blacks rose at least one percentile point in each category, and generally by two or three percentile points.

Blacks make up 8.4 percent of the county's students, but they account for 16.5 percent of students in the eastern area, which includes the Rte. 1 corridor and Fort Belvoir.

The 3.1 percent of county students who are Hispanic scored above blacks, but below whites. The 7.9 percent who are Asian scored at about the same level as whites, although the Asian 11th grade scores were lower.

Spillane, like officials in other school systems, cited the blacks' lower income and teachers' lower expectations for them as two explanations for the lower minority scores, but said they are no excuse. "Every child can learn," he said.

After a 1983 report showing that minority students lagged behind whites in test scores, enrollment in advanced courses and every other measure of achievement, former school superintendent William J. Burkholder encouraged the county School Board to make minority gains a top priority. It approved a school-by-school program that began last fall.

This year, the county will spend $2.1 million for the program, most of it for more staff and teachers in 22 elementary schools with high numbers of minority and poor students. Money also will go for summer programs, Head Start for preschool children and grants for individual projects.

"I wouldn't say enough is being done," said Robert E. Frye, the former minority representative to the School Board. He said the schools made "an outstanding start," but "just the addition of extra resources in a few schools doesn't address the problem."

Educators said they will publish an update of the 1983 report this fall, as well as school-by-school profiles on minority achievement to encourage progress.

Spillane said, "I'll probably be embarrassed next year and be hiding if scores don't show significant growth."

Frye and Frank Francois, who succeeded him on the 10-member School Board July 1, said more must be done in the county's intermediate and high schools. However, Francois said, "The board is on record as doing all that's necessary to close the gap."

School Board Chairwoman Mary E. Collier said she is pleased by the small improvements in black scores, but added, "The danger is in expecting a quick fix."