REFLECTING THE widespread belief that schools need to be made more accountable for the quality of education they provide, the Senate has passed legislation to expand an 18-year-old test of national school quality -- the National Assessment of Educational Progress -- and, separately, to create a national "Optional Test of Academic Excellence" for 11th-graders. Does the country need more national testing? The answer depends on what kind.

The well-regarded National Assessment aims to evaluate the schools. It doesn't test every student, but instead picks a small, reliable sample -- 120,000 students now, 700,000 under the proposed expansion -- and publishes the results as general indications of how groups and regions are doing overall. The Senate's legislation would allow it also to compare states with one another, something state education chiefs were reluctant to do 18 years ago, when the test was launched. What the National Assessment doesn't do is give parents, students or superintendents any idea how a school is doing. But it makes information available on what is being taught and how well. The test is put together by professionals under a process meticulously designed to keep any politician or agency from dictating a de facto "national curriculum."

The proposal for an optional, nationwide 11th-grade test takes no such precautions. It merely authorizes the secretary of education to choose or develop a test and asks him to report back to Congress when he comes up with one. Sen. Claiborne Pell, who first proposed such a test 20 years ago, thinks it would a) offer a comparative measure of school quality, and b) identify academic talent in students "who might not otherwise be recognized." Sen. Pell made the test optional because the idea of a national, mandatory testing program set off all kinds of alarm signals among educators and politicians alike.

Unfortunately, an optional test can't do either of the stated jobs. Tests taken by only the more highly motivated students don't reveal much about their states or schools overall, any more than average scores on the Scholastic Aptitude Test -- taken only by the college-bound -- tell you how a school district is serving its weaker students. Those same weak students are highly unlikely to walk into an optional test one day and reveal hitherto unsuspected skills.

The strongest argument against rushing into a new test of this kind isn't the philosophical one. Some opponents fear any national test would erode crucial local control of schooling. But in fact, and somewhat paradoxically, a firm agreement on what sort of education the schools ought to produce (as opposed to how they ought to produce it) can leave school districts much freer to reach that result any way they see fit. The expanded National Assessment may well help meet that goal. But adding any new test has its hazards -- not least, leaching away even more time from a school year already heavily burdened with tasks other than teaching and learning. And if a test is carelessly designed or unevenly administered, the scores students make on it can do them as much harm as good.