Black students in Montgomery County public schools scored below their white classmates on a variety of aptitude and achievement tests administered last year, county school official said yesterday.

According to a new computer print-out based on the 1977 county-wide testing of students, blacks lagged behind their white counterparts by the equivalent of anywhere from eight months to slightly over two school years, according to schools Superintendant Charles M. Bernado.

Bernado said that the test differences "very possibly" may be due to cultural biases and lower expectations on the part of teachers who teach black students.

As a result, teachers may demand less of blacks in the classroom than is demanded of white students, resulting in less incentive for blacks to achieve their correspondingly lower test scores, officials said.

Fifty percent more county black students were found in the lowest four rankings of student academic ability tests than would be expected.

When the scores of the county's black and white students were compared to those of students nationwide, the scores of the country's black students fell almost always below the national average while those of the county's white students were consistently above it, the study showed.

Howere, the scores for Montgomery County's students as a whole were above the national average, Bernado said he "inferred that our black youngsters score higher than black youngsters nationwide."

One chart presented by Bernado to reporters yesterday combined scores for reading comprehension, language, work study skills, and math on a grade by grade basis for both black and white students. For example, in the third grade, the national norm was listed at 3.7, meaning the national achievement norm for third grade students was three years and seven months (school year is considered to be 10 months). In Montgomery County, blacks had a composite score of 3.54, or about six weeks below the national form, while whites had a composite score of 4,43, or more than a year above it.

For the ninth grade, the national composite norm was 9.6; in Montgoemry County blacks had a composite score of 7.83 or nearly two years below the norm, while whites had a composite score of 9.78, or slightly above the national norm.

THere are apparently no national studies of black school students that used precisely the same factors as the Montgomery County study, officials said.