Montgomery County schoolchildren performed at about the same level on standardized tests they took the last school year as they did in 1976-77, school officials said yesterday.

Black students' tests scores, which lagged behind those of whites last year, are catching up, according to the study released yesterday. Superintendent Charles M. Bernardo said black students made gains that were "moderate but statistically significant."

An analysis of how much black test scores have improved will be completed within two months, said Steven Frankel, director of the Office of Educational Accountability for the school system.

Montgomery County pupils' scores on ability and achievement tests, given to third, fifth, ninth and 11th graders, already rank among the highest in the state and will probably continue to do so this year, Frankel said.

Commenting on the lack of dramatic improvement on the tests, Bernardo said, "It's very hard to get better when you're already among the best." On the three tests that make up the Cognitive Abilities Test and the 12 sub-tests that make up the Iowa Tests of Basic Skills, third and fifth graders improved the most. The average score for all the tests for third graders was at the 79th percentile. That means Montgomery County third graders scored better than 79 percent of the third graders who took the tests across the country.

Junior high students'scores were down somewhat on some tests.