THE D.C. BOARD of Education has decided that, beginning in 1988, new teachers will have to take a battery of tests to help determine their competence as classroom instructors. This is good news. School system officials say they want the test to focus on the teachers' knowledge of subject matter, but beyond that little is certain. The test has not yet been developed, and school system officials are unsure whether it will be used to validate hiring, as a test for certification or as a test for tenure.

Still, it sounds like a step in the right direction. Thirty-one states across the country, including Maryland and Virginia, already test new teachers before they are allowed into the classroom. Most states use the standard National Teacher's Exam, while 10 others have developed their own tests.

There are reasons why a good test for new teachers is of greater importance to the D.C. schools and should have been developed sooner. More than 60 percent of the school system's 5,700 teachers have taught for 15 years or more and will be reaching retirement age in the next five to 10 years. A new crop of teachers hired in the late '70s would have meant that more seasoned instructors could have eased that transition, but many were laid off during a period of budget austerity.

The school system's responsibility is to its students, and it must make a reasonable effort to ensure that incoming teachers are fit to teach. School system officials say they have not decided what will happen to teachers who do not score well. D.C. officials are right in saying that the test should not be the sole factor in deciding whether a teacher is hired. Such a test would not take account of the zeal and imagination a prospective teacher might bring to a classroom. And a teacher who did not perform exceptionally well on the test conceivably could still be a good teacher -- with the addition of some supplementary course work and another crack at the test. But this said, the fact remains that those who fail the test outright or do remarkably badly on it should not be permitted to teach in the schools unless they can reverse those results and make the grade. If passing grades are required for student participation in sports, it is surely not unreasonable to insist on them also for the right to teach.

It is good that the decision to test new schoolteachers has the support of the Washington Teachers Union. It has that support largely because the board decided it would not test veteran teachers, who already receive an annual performance appraisal. We hope that the teachers union will support a tough examination, not an easy one. The union would do well to recognize the need to develop tougher standards for veteran instructors. Weeding out incompetent teachers is the goal, and it is an essential one.