Some of Fairfax County's angriest parents were on their best behavior this week when the school board opened the first of three nights of public hearings on a proposal to close eight elementary schools.

However, it was not quite as calm as at the second hearing Tuesday night when members of the audience toted placards and periodically interrupted speakers with applause. The third hearing is at 7:30 tonight at Luther Jackson Intermediate School.

The hearings opened with Supervisor Sandra Duckworth, a Democrat from the Mount Vernon District, where two schools have been proposed for closing. Duckworth was one of four supervisors scheduled to address the board.

Duckworth told board members she was there not only as an elected official but also as a mother of three children. She suggested that instead of closing schools, the board close the Area I administrative office and move the employes to vacant classrooms.

Duckworth said that she was "disappointed" that the school closing process and its citizen advisory procedure had been "subverted by inconsistent . . . data" about the projected growth in the county.

Del. Gladys B. Keating (D-Fairfax), the next speaker, topped Duckworth's claim to motherhood by noting that she not only was a member of the General Assembly but also the mother of five Fairfax County students.

Keating charged the school staff with ignoring the report of its citizens' advisory committees and causing the erosion of public confidence in government: "I doubt that the staff even opened the first page of this (citizens' report on school closings) report. I hope you have."

Also on hand for the first night of hearings, but making no appeals to the maternal instincts of the board, was Supervisor Joseph Alexander (D-Lee), who began a campaign several months ago to keep Lee District schools open.

In urging the board to reject school closings in the Lee District, Alexander repeated his contention that the Lee area has more undeveloped land than anywhere else in the county and brought along site plans to show that some growth had already begun.

Alexander said he sympathized with the "unenviable" position of the board, but added that in all of his years in public office he had never heard so many people saying the same thing.

"They don't want any schools closed," Alexander said. "Listen to the people, I think they know what's right."

With 45 speakers slated to address the board each night, it was inevitable that several themes would come up again and again. Most speakers agreed that the proposal by Superintendent L. Linton Deck ignored recommendations of board-appointed citizen advisory groups. They also suggested that Deck's proposal was based on factual errors, primarily in projecting population trends and financial savings.

"In the hours and hours I have spent on this study," began parent Bill McGee, "it has become clear to me that (the) school staff erred in this matter and have acted like typical bureaucrats caught in a mistake -- that is, with much blustering and double-talk defending an indefensible position."

Welby A. Smith, president of the Hollin Hills PTA, called the superintendent's proposal a "shoddy performance. I suspect that after all of the time and money wasted on this expensive exercise in futulity, it has finally dawned on the superintendent and his staff that there are no compelling reasons to close these schools and they're ashamed to admit it."

Even though all speakers opposed school closings, some conceded that a few schools might have to be closed.

One group of parents suggested that neighboring schools, not theirs, be closed. Others said they would support the closings, but believed the decision shold be postponed for more study.In line with that, many speakers urged the board to delay the closings until the full effects of the new subway line are determined or until information from the 1980 census is available.

Two communities -- Hollin Hall and Wilton Woods -- presented informal census counts of pre-schoolers in their area, and contended that the results refuted claims by school planners that population was declining in their areas.

The Hollin Hall School Action Committee brought along its lawyer, who carefully outlined the legal weaknesses in the school board's closing procedures without directly mentioning a lawsuit.

Neighborhood schools and the cohesion they bring to a community also was a recurrent theme. Speakers repeatedly questioned the wisdom of closing neighborhood schools and busing the children to other schools during a time of dwindling energy sources.

"The superintendent is asking you to . . . disrupt the lives of more than 600 elementary school children (in Hollin Hills and Hollin Hall Elementary Schools), most of whom now walk to school, and bus them past the shuttered buildings you close to new schools . . .," said PTA President Smith.

One of the more emotional testimonies came from Aurora Reagan, a U.S. citizen originally from Peru.

In halting English, she told the board that many of her fellow countrymen dreamed of coming to the United States for a fine education, and she could not understand why the board would want to close schools.

"I think this superintendent's recommendation is big mistake," she said as she was drowned out by applause.