The Arlington School Board has decided to allow principals to exempt one of every 19 "classroom teachers" from being laid off or transferred if they are "essential to the effective operation" of the school's program.

The new policy gives principals the leeway to protect teachers who coach a sport or sponsor another extracurricular activity, such as debate club, as well as teachers who have "special skills" in their academic fields. Principals must justify all exemptions to Superintendent Charles E. Nunley.

Currently, teachers with the least seniority are laid off or reassigned first--a procedure most of the county's teachers argued vehemently in favor of retaining during several meetings on the issue in the past year.

Teachers maintained that, under the new policy, teachers with seniority in their academic fields could be laid off in order to keep less experienced ones who had agreed to coach. Board members, however, said the new policy actually would enable principals to keep exceptional teachers even if they have little seniority. Board member Simone (Sim) J. Pace also argued that good teachers and coaches are not mutually exclusive terms.

The board's vote was 3 to 1, with Torill B. Floyd dissenting and Evelyn Reid Syphax absent. Member Michael E. Brunner, who proposed the policy change as a way "to protect the educational program for our children," pointed out that very few teachers would be affected. Pace and Chairman Claude M. Hilton voted with Brunner.

Marjorie McCreery, executive director of the Arlington Education Association, which represents 90 percent of the system's nearly 970 teachers, said 28 teachers probably would be affected this year, depending on how Nunley defines "classroom teachers."

At Brunner's request, the board deferred action on a last-minute policy change suggested by Nunley, which would have extended the protection concept to vacancies that develop during the school year.

The AEA argues that Nunley's proposal would, in effect, allow principals to change the job description for a vacancy so that an applicant's willingness to coach or sponsor a club activity would be considered along with the applicant's academic credentials. Under the proposal, McCreery said, an applicant for an opening in the math department, for example, could be required to coach wrestling in order to get the job.

The AEA also contended that the protection of vacant positions would allow principals to hire from outside the system, bypassing county teachers who were laid off and who ordinarily would be recalled in order of seniority as openings occurred.