More than 300 Fairfax County parents crowded the cafeteria of Robinson Secondary School last week, expecting to hear the school board announce that their children would be moved to different schools next year.

Instead, the parents listened for two hours to school planners use such words as "planning units" and "centroids" to explain a complicated computer system. The board expects to use the system to help decide what schools to study for closing next year and how to reassign students.

Many parents left the meeting saying they were disgusted and confused after listening to the computer jargon and hearing no decisions from the board. Rumors of impending closing studies and massive boundary changes next year apparently had spread among PTAs and had attacted the parents to the special school board workshop.

One parent, suspicious about school board motives, said, "They'll change boundaries in such a way to empty a school then say it has to close."

Another parent who seemed wary of using the computers said, "The computer will become the scapegoat for all their decisions."

Board members appeared no more secure than the parents about entering the computer age of decision-making. They also expressed reservations about using computers to recommend changes in attendance areas and to target specific schools for closing.

"The problem I always have is a fear that this approach implies that decisions can be turned over to the computer," said School Board Chairman Rodney Page."We have to take steps to ensure that playing with this thing doesn't leave us without any flexibility to make decisions."

What those steps will be is unclear. Board members also say they are unclear on how to use the data supplied by the computer, although they discussed using such data as a guide to school closing studies - a process that begins next fall.

Despite the crowded schools in some areas, overall enrollment for the school district has declined. That trend prompted the board to adopt a new school closing policy two weeks ago. Under the policy, the board would examine an elementary school for closing if its enrollment falls below roughly 350 pupils, if the cost per pupil is significantly higher than the county average, and if the school is a candidate for major renovation. There are more than 25 elementary schools that now meet the criteria for closing studies.

The school administration has develpoed three computer programs to show options for redistribution of pupils throughout the county. The programs are designed to assign students to schools closest to their homes and to even out the unbalanced school attendance areas. The redistributions also would leave many schools underenrolled.

To conduct the computer studies, the county was divided into 3,013 geographical "planning units" with 25 to 50 pupils in each unit. The distance between the closest school and the center (or "centroid") of the planning unit was then measured. Under guidelines developed for one study, a student would attend the closest school and travel no farther tthan 7.5 miles to get there, and each school would be filled to no more than 85 percent capacity. Using these guidelines, the computer determined that 29 elementary schools would be less than half filled.

For example, Beech Tree Elementary School would have only 96 students, 24 percent of its capacity, under these guidelines. The school board is expected to develop additional guidelines for the computer studies on redistribution of students. Those guidelines, for instance, could specify that pupils not cross any major highway in traveling to school and that a school be racially balanced.