Montgomery County public school students, for the third year in a row, have scored slightly higher on academic achievement tests than the previous year, while more than 75 percent of county students continued to score well above the national norm.

In addition, results from the California Achievement Tests released yesterday indicate that the gap in test scores between white and black students continued to narrow in the county, while female students scored slightly higher than male students in almost all categories except math. In math, both boys and girls in the third, fifth and eleventh grades scored the same, with eighth grade girls testing slightly higher than boys at the same grade level.

The results of the tests, which have been given every fall since 1980, were hailed by school administrators as indicating that the county schools still are able to provide the type of education a predominantly affluent population demands while also meeting the schooling needs of the increasingly urbanized lower county area.

The scores "show an upward trend from a level of achievement that was already among the highest of any large American school system," said Superintendent Edward Andrews.

The tests, which examine reading and math ability, were administered last fall to 25,700 third, fifth, eighth and eleventh grade students. Montgomery third and fifth graders scored in the 81st percentile nationwide, while eighth graders scored in the 79th percentile and eleventh graders in the 76th.

Students scoring in the 50th percentile are at the national average, with half of those who took the test below and half above their score.

In reading, Montgomery third graders scored in the 74th percentile, two percentage points higher than last year's third graders. Eighth graders dropped a point to the 77th percentile, while fifth and eleventh graders remained at the 77th and 73rd percentiles, respectively.

In math, third graders jumped three percentage points to the 80th percentile, fifth graders jumped two points to the 79th percentile, eighth graders remained at the 79th percentile and eleventh graders jumped one point to the 74th percentile.

No comparable scores were available from Northern Virginia school districts because they do not administer the California test. Prince George's County has not yet released its figures.

Some of the largest percentage gains in the three years in which the tests have been given were scored by third and eleventh grade black students in reading, although eleventh grade black students were the only group to score below the national average in reading and math. In 1980, the first year the tests were administered, black third graders scored in the 44th percentile in reading; this fall they scored in the 52nd percentile.

Steven Frankel, director of the school system's Department of Educational Accountability, attributed some of the continued upward trend to the increasing enthusiasm of some teachers for preparing their students for the specialized tests. He said some schools buy test preparation kits.