The Fairfax County School Board has approved a $3 million contract to renovate Marshall High School, soundly defeating a proposal to delay all such projects until a study on declining enrollment countywide is completed sometime next year.

The vote last week marked the second time this year the School Board has opted to continue with its voter-approved capital projects program when confronted with charges that to do otherwise would call into question the board's "credibility."

"The needs of the children--that is what the 10-year commitment of this board to renovation as well as construction is all about," declared board Chairwoman Ann P. Kahn.

The vote, which took place before a highly partisan crowd of about 300, virtually ensures the board's approval a few weeks from now of a $4 million contract for renovation of W.T. Woodson High School. Funds for both projects were included in a November 1981 bond referendum.

In a memo to School Board members, Superintendent William J. Burkholder warned that delay of the Marshall and Woodson renovations might increase their costs by as much as a total of $1 million. But Burkholder's memo concentrated first on the credibility question.

Delay, Burkholder wrote, "could raise concerns regarding the commitment and intent of the School Board."

It was an argument also voiced by some of the dozen people who spoke before the board last week.

"You are the same board" that asked for the bond referendum, Marshall parent June M. Jaeger charged. "How can you keep your credibility if you change your mind now? Who will believe you, particularly in the older sections of the county, the next time you propose a school bond referendum?"

"Where do we stop?" Annandale board member Laura McDowell asked at one point. "If we reverse ourselves less than two months after approving the 1984-88 capital improvement progam, we'll open a Pandora's box" of opposition.

The proposal to delay construction was made by Springfield representative Toni M. Carney. Carney represents a rapidly growing section of the county that is less likely to benefit from the renovation of older schools in areas of the county where population growth has leveled off or decreased.

"I wonder if we'd be hearing about a moratorium on renovations if Mrs. Carney represented the Mount Vernon or Lee districts," asked Mount Vernon member Gerald A. Fill, whose district is declining in population.

Fill's implication that Carney was playing population politics harkens back to a previous acrimonious board debate. The last time "credibility" clashed with fiscal conservatism it involved a West Springfield area project. In that instance Carney came down on the credibility side, voting in favor of new construction rather than delay.

In that debate in January, the board voted to begin construction of Braddock Park Intermediate School in fiscal 1984, despite a proposal by the administrative staff to delay it for two more years while the school-age population in the Pohick watershed increased.

At that meeting, Board of Supervisors Chairman John F. Herrity and Springfield Supervisor Marie Travesky urged the School Board to stick to the original schedule to protect the credibility of future bond issues.