In a marathon session punctuated by yawns and giddy laughter, the Fiarfax County School Board voted last week to approve a new set of guidelines for elementary-school closings.

In spite -- or maybe because of -- the 10-member board living had a month to ponder a set of staff proposals for revising the policy, the school board spent 2 hours and 40 minutes debating amendments to the policy.

Some of the highlights of the session -- during which a crowd of nearly 100 persons dwindled to a handful -- included spirited arguments over the use of the words "may" and "shall" and the board's first (and second) tie votes since Virginia's General Assembly passed a law requiring that a tie-breaker cast the deciding vote.

The school board avoided summoning tie-breaker and ex-board chairman Rodney F. Page to resolve the dispute over a proposed amendment by recessing for five minutes to allow members to caucus on the issue. To avoid the appearance of arriving at secret decision, vice chairman Roger Teller advised board member (who would violate the state's open-meeting law if three or more discussed the issue) to "keep moving (around)."

When the meeting was called back into session, chairman Ann P. Kahn offered to change her vote.

One staff proposal for modifying the school-closing guidelines involved omitting "quality of education" as a criterion. This apparently resulted from a lawsuit last summer by Edsall Park Elementary School parents who argued that their children's school should not have been closed by the board because the pupils there had tests scores higher than the county average. The parents lost the case, and the school remains vacant at the corner of edsall and Backlike roads.

Although several members of the board said "quality of education" was difficult to define and should therefore be eliminated, board member Eltse Carter convinced her colleagues to retain the original wording.

"I can't define half of what we do here," Carter said in exasperation. "But to replace 'quality of education' with 'instructional flexibility and continuity' (the proposed wording) defies my sense of reason -- even at this hour."

In the end, both phrases were included.

In other matters, the board awarded $32,226 in cash prizes to 36 county schools that had conserved energy during the 1979-80 school year.

In addition, members of the pimmit Hills community asked the board to close their local elementary school and incorporate it into neighboring Westgate Elementary School because of declinging enrollment. The move apparently was intended to forestall possible inclusion in an agonizing three-month-long closing study like the one last year that resulted in the close of seven elementary schools. The school board staff is expected to respond to the Pimmit Hills request at the board's next meeting Dec. 4 at the Robinson Secondary School cafeteria.