VIRGINIA'S SCHOOLS have now begun to report the results of the competence tests for 10th graders. Those tests are an immensely promising innovation, signaling a change in the definition of schools' reponsibilities. In the early 1950s, when hardly more than half of all children graduated from high school, schools throughout the country began working with skill and energy to keep children in the classroom. There was a broad national consensus that a high school education ought to be not merely for an elite, but for everybody.

But in the interest of holding a maximum number of children in the classroom, most school systems suffered an erosion of the minimum standards for a diploma. That was unfair to the community, to employers and, above all, to youngsters who were being misled regarding their own training and capacities. The competence tests are a device to lift that basic standard -- but without driving anybody out of school.

Tests like these are double-edged. They identify the children who can't handle numbers or the written language. But they also identify weak curriculum. Alexandria, where one student out of five failed the test, provides a good illustration of the way the process is supposed to work. The school system has already identified the most often missed questions and the points in the children's education where they should have learned the answers. If a large number of 10th graders can't handle a mathematical concept routinely taught to fifth graders, that revelation invites revision of the fifth-grade arithmetic curriculum.

For the youngsters who failed the math section, the Alexandria schools have already made up individual tutorial packets. For those in trouble on the verbal section, additional programs are to getr under way after the holidays. The children who failed get another crack at the test in April.

Incidentally, Alexandria is now tightening up the rules in otehr respects as well. In the past, a youngster could get a diploma without much more than 12 years' attendance in school, and sometimes the attendance toward the end got pretty ragged. This year, stiff new attendance rules have gone into effect, and truancy has already dropped. But there's also the question of the children -- to take a poignant example, the handicapped -- who attend school faithfully with great benefit but, for reasons that are neither their fault nor any school's, will never pass the tests.In the past, everybody got the same diploma. But Alexandria's school board has decided, beginning in 1981, there will be three different kinds of diplomas -- honors, regular and a special category. Any child who stays with the course is entitled to something to show at the end of it. But It's also necessary to distinguish between those who can meet the full requirements and those who can't.

Success nd failure rates on the tests will vary from one school to another, just as the communities vary. How many ought to fail? A city like Alexandria serves a very wide range of children, including a good many who are foreign-born and a good many more who come from very poor families. A failure rate there of 20 percent on the first round seems just about right. The students who fail have a claim for special attention in the months to come. To fail many more than one-fifth would spread these claims too widely among children who do not really need that kind of help. But if few failed, it would be clear that the tests were too easy to fulfill their basic job of diagnosis. These tests have a valuable contribution to make to the education of Virginia's children, and the experiment is off to a hopeful beginning.