The Fairfax County School Board may have to delay several proposed construction projects by a year or two in order to answer objections that its five-year building plan endangers the county's AAA bond rating, a top school official said yesterday.

County Executive J. Hamilton Lambert has told the School Board that its recently approved five-year capital plan would require the sale of $73 million more in bonds than allowed under Board of Supervisors guidelines designed to preserve the county's good credit rating.

Assistant School Superintendent Alton C. Hlavin, who is preparing a proposal to answer Lambert's objections, said the school system cannot pare the proposal by $73 million "and meet our urgent requirements."

But, he said, "We're going to aim in that direction and there will be some substantial reduction by shifting some projects to the fourth and fifth years" of the five-year program. That might mean beginning construction on some projects a year or two later than now proposed, so that the projects would be paid for after 1991, Hlavin said.

"We have been promised we can have every one of those projects," said School Board Chairman Mary E. Collier.

The School Board voted Jan. 23 to approve $196 million in additional construction for 1987-91, including seven new elementary schools, an intermediate school, 11 elementary school additions and three special education centers.

The Board of Supervisors must approve the proposed spending, and most of it would have to be authorized by voters in bond referendums. School Board members said the construction is needed because of a baby boom in the county and the addition of special programs requiring more classroom space.

Lambert told School Superintendent Robert R. Spillane of his concerns in a memo late last month, and the two, along with Collier and other top officials, met to discuss the issue last Thursday.

"I told them it's certainly not within my judgment or purview to make decisions relative to specific projects," Lambert said. "My concern was the amount of cash they would need each year."

The Board of Supervisors' policy aims for a target of not more than $80 million per year in total sales of bonds, including those for roads, schools, parks and other needs, and an absolute limit of $100 million annually. The supervisors also set a five-year ceiling of $400 million.

The School Board construction proposal would put total county bonding $73 million over the five-year limit, Lambert said. In addition, the proposal would exceed the $80 million target in some years, officials said.

The School Board is expected to discuss Lambert's demands at its regular meeting tomorrow. One member, Laura I. McDowall, said she will oppose any change in the construction plan, contending that all of it is needed.

Hlavin acknowledged that postponing some construction would add to the plan's total price tag, because building costs are rising.