The gap in performance between black and white students in Montgomery County schools on a nationally recognized achievement test has narrowed since 1980, according to a report released yesterday, but the gap between Hispanics and whites has gotten wider.

All grades taken as a whole registered improvement for the sixth year in a row.

The students in grades 3, 5, 8 and 11 were given the California Achievement Test last year, and their performances were judged against the performances of students nationwide on the same test. The test measures students' knowledge of reading, language and mathematics.

All grades in Montgomery scored in the 80th percentile or higher in 1985, meaning better than at least 80 percent of students in the country as a whole.

In the 11th grade, black students scored in the 54th percentile, while whites scored in the 82nd percentile. The 28-point gap was nine points less than in 1980, according to the school system's figures, when blacks scored in the 39th percentile and whites in the 76th percentile. The gap decreased similarly at other grade levels.

Hispanic 11th graders scored in the 59th percentile in 1985. Their score remained the same as in 1980, while the whites' score rose, thus widening the gap, which was mirrored in other grades.

The report showed that the black-white gap increases as students get older. That alarms Hanley Norment of the Montgomery County Chapter of the NAACP.

"I am pleased there is some progress," Norment said. "But I'm really troubled that consistently there is a showing that the longer black students are in the public schools the greater the gap becomes . . . and it ought to be just the opposite."

Black students' scores have increased by 7.5 points over the last six years and 1.5 points over the last year.

Hispanic students gained 0.5 points over the last year and 1.25 points over the six-year period. Asian students' scores, which are consistently high, have gone up 3.5 points over the six years and 1.25 since last year. Whites increased 0.5 over a year ago and have gone up 5.25 points over the six years.

"Average scores for all these groups . . . are at or above the overall national norm average," said county School Superintendent Wilbur Cody. "I believe this validates the programs we have set in motion on behalf of minority students, while realizing that we must maintain our efforts to raise student achievement at all levels."

Systemwide, third graders did better than 86 percent of students nationwide, fifth graders scored better than 87 percent of students nationwide, eighth graders scored better than 84 percent of students nationwide and 11th graders scored better than 80 percent of students in the national sample.