The percentage of Montgomery County ninth graders who can read and compute rose slightly last school year over the year before, according to preliminary results of the Maryland Functional Reading and Mathematics test released yesterday by the school system.

Of ninth graders who took the test last spring, 7,745, or 98 percent, passed in reading and 7,776, or 79 percent, passed in math.

The previous year, 7,747 ninth graders, or 97 percent, passed the reading portion of the test, and 7,847, or 78 percent, passed the math portion.

Test scores are still being compiled for two other competence exams administered by the state Department of Education -- writing and citizenship, a newly required portion that tests students' knowledge of government.

Hispanics and blacks, two groups that historically have lagged behind countywide achievement and that have become the focus of an intensified effort by the school system, did slightly better this year but still did not reach the performance level attained by the system as a whole.

The achievement of Asian students, which also has prompted concern from educators, increased slightly in math -- in which they scored higher than other groups -- but decreased in reading.

The math achievement level of county ninth graders, traditionally higher than the state average, experienced a major increase in 1983.

That year, 78 percent of all ninth-grade students taking the math test passed, or 13 percentage points above the previous year. In reading, 97 percent of the students passed, one percentage point higher than the previous year.

School officials said that although they were pleased with the slight gains this year, they could offer no reason why history did not repeat itself in showing significant increases in the number of students passing the math test.

"Your guess is as good as mine," said Jim Myerberg, testing coordinator for the system.

"Whether 78 or 79 or 80 percent is the point in mathematics where we're going to level off, we don't know . . . . It's difficult to improve in reading when you are already in the 98 percent level."

School administrators and principals will be meeting today to discuss the achievement levels and what steps schools can take to prepare the students for the tests, all part of a state-mandated system to strengthen graduation requirements.

According to state law, students must pass the reading test to receive a diploma.

Passing the math test will be a graduation requirement for students graduating in 1987.

Students who fail the tests have two opportunities a year in 10th, 11th and 12th grades to retake the test.

This year's competence rate for the county is the best in both categories since the tests were instituted in 1977, Montgomery school officials said.

Statewide figures were unavailable yesterday. Last year, 93.4 percent of ninth-grade students in Maryland passed the reading test and 76.2 percent passed the math test.

The only decline among minority students showed up in among Asian students passing the reading test, which dropped one percentage point this year, to 92 percent. However, a decline that small is considered statistically insignificant.

Lois Martin, associate superintendent for instruction and program development in the system, said she was pleased at the overall increases in minority achievement.

"What it proves is these kids can do these things -- if we expect it of them and make it known that it is expected of them," Martin said.