HOW VALUABLE IS every scheduled hour of instruction in Washington public schools?

That depends on who's doing the instructing; but it isn't worth a thing when nobody's there to teach. And that, according to a study ordered by the D.C. school board, seems to be happening far more often than it should.

It happens most on Mondays and Fridays, especially in the secondary schools, where an average of 5.6 to 7.6 percent of the teachers are out every week, according to projections from the survey of 12 city schools between December 1984 and last spring. This absenteeism rate is more than double the national average for nonvacation absences, according to the U.S. Bureau of Labor Statistics. Worse still, the study says that usually no substitutes are assigned to replace absent teachers.

What's going on here? Is the general health of this city's teachers really that poor? If so, perhaps these chronically absent instructors should try other lines of work that don't take nearly such great daily tolls.

Yet, neither school officials nor teachers' union leaders seem all that concerned. The administrators point out that teachers earn 10 days of sick leave in a school year, and use, on average, about seven days. And listen to the explanation of Washington Teachers Union President Harold Fisher: many instructors are absent, he says, because "teaching is a high-stress and demanding job. In any such job, you would expect that there's going to be a tendency towards absenteeism."

Well, things are tough all over, but the vast majority of chronically well teachers who do show up regularly would indicate that the job can be done. As it is, the time of any D.C. teacher is precious enough, allowing little out-of-class availability for students. When that teacher can't be in class, a substitute surely should be -- and every effort should be made to produce a qualified stand-in. School board member Edna Frazier-Cromwell, who requested the study, deserves encouragement for her determination to reduce this absenteeism.

Sick-leave days are not, after all, automatic vacation days -- and those teachers who abuse this provision should be warned and/or disciplined. Without invading any teacher's privacy in health matters, principals and administrators might consider posting the number of hours each teacher has been absent from classes. It is a public matter, anyway -- and it could provide a useful start in weeding out those who are taking advantage of the system, of their honest colleagues and of the children entrusted to them.