ACADEMIC TESTS are a powerful instrument for tightening the standards of teaching, and learning, in a school system.But they are also a threat, not only to the youngsters who will take the tests but to the teachers and administrators who will be held responsible for the pattern of results. Any new program of testing raises large questions of fairness.The Montgomery County School Board is now considering a requirement for uniform countywide final examinations in all academic subjects in its senior high schools. In an article on the opposite page, two parents, Phyllis Brush and Ruth Harris, argue the case against this kind of test.

But there is also a case for proceeding with the tests. Equality of opportunity is never easy to maintain in a large system like Montgomery's, with 22 senior highs serving widely varying neighborhoods. In those parts of the country where the adults are most highly educated, parents typically keep both students and their schools under pressure to maintain a fast pace in the academic courses that lead to college admissions. But in other communities, where parents are less attentive, it's easy for a school to relax a bit and perhaps drop some of the more troublesome topics out of the curricula.

The purpose of a uniform countywide final examination is to set a uniform countywide standard for students and, perhaps more important, for their teachers. The opponents point out, correctly, that this kind of examination gives an advantage to those students in the fast courses, taught by the best teachers. But when high school seniors take College Board exams, those same disparities are inevitably revealed anyway -- and at a point when the schools no longer have time to do anything about them.

The specific proposal before the school board sensibly provides for a period of experimentation. The tests would be introduced gradually, probably beginning in the spring of 1981. For the first several years, while the results might be used by teachers in grading their classes, those scores would not be separately recorded on report cards or college applications. But they would be circulated within the school system as a guide to areas of academic strength and weakness.

Americans have recently developed a sharply ambivalent attitude toward academic tests in general. Everyone realizes that no two children start at the same point, and there is always some degree of inequity in this kind of rigorous comparison. But testing is essential to the kind of highly developed society of which Montgomery County is a notably successful example.

The objections raised by Mrs. Brush and Mrs. Harris deserve the school board's careful attention. But it is capable of responding to them and still proceeding with the new examinations.