The Fairfax County School Board, after weeks of intense lobbying by teachers, parents and politicians, last night reversed itself and voted to continue closing elementary schools as much as 2 1/2 hours early on Mondays.

Superintendent Robert R. Spillane's plan to extend Mondays to the same 6 1/2-hour school day as the rest of the week failed on a 5 to 5 vote, prompting raucous cheering, applause and even some tears from the more than 600 teachers who packed Luther Jackson Intermediate School, where the board meets.

"We did it! They listened!" shouted Maureen Daniels, president of the 6,900-member Fairfax Education Association, which led the campaign to defeat the plan. "This was the right way to go."

The vote was a major defeat for School Board Chairman Kohann H. Whitney (Centreville), the plan's leading champion for the past 18 months. Before the vote, she urged her colleagues to approve the plan because it would add the equivalent of three weeks of class time for the school system's 72,196 elementary students each year.

"I have 72,196 reasons for voting yes," she said.

Last night's decision represented a dramatic retreat for a board that once unanimously supported extending the Monday class day. Nearly a year ago to the day, the same board voted to approve the plan "in concept" but left details until this fall.

In the interim, the region's economy took a nosedive, forcing local governments and schools to cut budgets. Most board members who voted no last night cited the plan's $5.9 million price as unaffordable at a time when principals are trimming money for textbooks, field trips and other items, and face worse cuts to come.

Teachers opposed the plan on other grounds, claiming they would no longer be able to plan collaboratively, as they do now on Monday afternoons. Spillane's plan would have hired 135 teachers to provide planning time in smaller blocks scattered throughout the week.

Last night was the culmination of a massive campaign by the two teacher unions, which combined spent more than $8,000 for hats, buttons and signs. Before the vote, about 600 teachers rallied and made sport of Spillane's nickname, Bud, with chants like "Bud's wrong" and placards such as "Ban Bud's Blue Light Special" and "Nip it in the Bud."

A group of teachers from Greenbriar West Elementary School led the crowd in a rap song that declared their weariness of "dealing with a looney tune" and included the refrain, "Bud, you gotta get you a new attitude."

After the final vote, Mark Glofka, vice president of the smaller union, the Fairfax County Federation of Teachers, said, "It renews my faith in the board -- at least {in} five members."

In addition to the teachers, board members also faced pressure, both publicly and behind the scenes, from the county supervisors who appointed them and who face reelection next fall.

Board Chairman Audrey Moore, a Democrat whose largest financial supporter in 1987 was the Fairfax Education Association, strongly urged her two at-large appointees, Robert E. Frye and Nathaniel Choate, to vote no -- and they did.

Supervisor Thomas M. Davis III (R-Mason), who intends to challenge Moore in 1991, likewise pressured his appointee, Carla M. Yock, to vote no. But in the end, she supported the plan, saying it was "the right thing to do."

Board members Laura I. McDowall (Annandale), Anthony Cardinale (Springfield) and Armando M. Rodriguez (Mount Vernon) also voted to eliminate early closings. Board members Anthony T. Lane (Lee), Letty A. Fleetwood (Providence) and Joanne T. Field (Dranesville) voted against the proposal.