When the Fairfax County School Board reconvenes Sept. 4 after a month's vacation, it will be a very different body from the one that presided over the opening of school a year ago.

Four members will be gone: Chairman Rodney F. Page, Vice Chairman Ruth H. Dell and Smith resigned this spring, and Flack left the board after her election last fall as the county supervisor from the Dranesville Distict.

In their places will be four new members: Mary Collier, the old-timer of the newcomers, who replaced Falck last November after Falck's election to the county board; Carmin Caputo, who filled Pagehs at-large post; Gerald Fill, appointed to Dell's seat, and Eltse Carter, the most recent appointee, who was named last week to replace Smith.

At the helm of the board will be the new chairman, Ann P. Kahn, and the new vice chairman, J. Roger Teller.

What the board changes mean is anyone's guess at the point, but an intitial survey of the new members shows a growing disaffection for school closings and a new focus on looking more carefully at the wants of their constituents.

Two new members -- Fill and Carter -- seem likely to line up with longtime board member Anthony T. Lane in opposing any additional school closings, an issue the board is expected to face again next year as Fairfax County enrollment continues to decline.

If school closing opponents succeed in making a strong stand against more shutdowns, there may be a bitter fight on the board about the move.

No school issue was more umpopular in the community last year than the board's decision to close seven elementary schools because of declining enrollments.

Irate parents contested the school's population projections and cost-savings' estimates. Communitiy members also questoned the wisdom of having school board employes on the school closing study committees.

When the board overrode the public objections and closed the schools, it was Lane who stood alone as opposing all the closings. (Several board members had voiced mild objections to selected closings.)

Fill has promised to scrutinize any shcool closing plans for cost savings and proof that the quality of education would be enhanced. Fill also says he will urge citizens' study committees to look at ways of saving money other than closing schools. Carter appears to be an even more outspoken opponent of closings. At a public forum last month where several school board candidates were questioned by residents, Carter told the audience she would be hard pressed to vote for any school closings.

"The last thing I would do is close a school," she said. "There is something very precious about a small school where the kids know the teacher and the teacher knows the kids."

At his first board meeting last month, Fill emerged as something of a maverick when Superintendent L. Linton Deck and other board members failed to convince him that more than $200,000 should be spent on renovating the auditorium at Luther Jackson Intermediate School for a new board room. While other board members voted to appropriate the money, Fill abstained.

Fill said he objected to allocation on several points. He says that although extensive studies may have been done in the years before his appointment, he is not convinced that all alternatives to the costly renovation were investigated. He also objected on the matter of timing. " . . . It came on the heels of two school closings in my district," he said after the meeting. "I didn't feel it was appropriate for me to signal to my constituents that I was not aware of the trauma they had been through . . . which after all was done partly on the basis of cost savings."

Another hotly debated issue -- sex education -- promises to resurface sometime this fall. In the past, the school board has generally been in favor of a more liberal program.

Opponents of expanding the current program lost their staunchest supporter on the board when Robert Smith resigned. With Smith gone, foes of sex education will probably look toward at-large member Gary Jones for support. Last year, Jones indicated a reluctance to broaden the current program to include subjects such as abortion and birth control, which he says touch on the religious beliefs of many citizens.

Chairman Kahn is on record as favoring liberalized sex education. The new members, however, say they need time to study the existing program.