HOW MUCH time in a school day does a teacher spend actually teaching? As some of the current studies of American schools have observed, the incursions have been growing steadily more severe. It's not unusual for the usual daily paper work to take 15 minutes of a 55-minute high school class. Some schools assign teachers to monitoring halls and lunchrooms. In a typical elementary class, there is a continual coming and going as special programs take children out of the room and return them. To the teachers, it often seems that teaching time is the remnant that's left when all the administrative demands have been met. This shrinkage of usable time has become a serious constraint on the schools' performance, and it deserves more attention than it's getting.

There's an insidious habit of mind, well established among school boards and administrators, of regarding teachers' time as free. If the teacher is on the premises, that meets the legal and bureaucratic requirements; the diversions imposed on that teacher are invisible--at least until the students' achievement tests are scored. An hour in the classroom counts as an hour of teaching, unless someone actually comes in with a stopwatch. But occasionally people with an interest in this subject have come with stopwatches.

Elementary school children, it appears, commonly spend as little as one-fourth of their school time actually working on the basic subjects under the guidance of the teacher. Robert L. Canady of the school of education at the University of Virginia has been studying the use of class time, and cites cases where schools have raised achievement scores dramatically simply by more careful scheduling. That means cutting down on the disruptions, giving the classroom teacher more undivided time with the children, and providing more time for the teacher to work with small groups. That benefits in particular the children in the bottom half of the class, who most need strong direction from the teacher--and who suffer most from interruptions.

In recent months, various experts on the subject have recommended longer school days or school years. Both ideas are questionable. It would be preferable by far to make better use of the class time already available. Most school officials seem not to be fully aware of the extent to which the encroachments have gone in recent years. School boards and superintendents who complain about a lack of adequate resources might want to take a careful look at the ways in which typical school routines waste the most precious resource of all, time in which to teach. If any school board wants details, there are a good many classroom teachers ready to provide them.