Fairfax County's school superintendent and major teacher union reached a last-minute contract agreement yesterday that union President Mimi Dash said partially satisfies teacher criticisms of the county's merit pay plan.

The pact, which was drafted during a bargaining session that began Sunday and ended shortly before 6 a.m., will be put to a vote by the 6,600 members of the Fairfax Education Association at a date yet to be determined, Dash said.

Union leadership will recommend that members approve the 1988-89 contract, but Dash would not predict whether it will pass because she said implementation of the program, which bases teachers' raises on performance ratings, so far has corroded teacher morale.

Of the contract, she said, "We have reached some accommodation . . . . We obviously didn't get everything we wanted."

The contract that was polished off yesterday includes an average 8.8 percent raise for county teachers, whose average salary is now $33,282. The increase had been agreed to last year when the FEA and Superintendent Robert R. Spillane announced a sweeping $110 million pact under which the union endorsed merit pay in return for a 30 percent total raise over three years and a voice in implementing the details.

Dash said the union gained ground on four of its major concerns in the proposed new contract. She said it won promises of more detailed explanations of the standards underlying the five job ratings, more clearly defined procedures and deadlines for evaluators, and more assistance for teachers who want to improve their job ratings, Dash said.

She said Spillane promised to reconsider his opposition to letting teachers who are given the middle rating -- "effective" -- apply for 10 percent bonuses; the superintendent wants only teachers given the top two ratings to be allowed to apply.

Spillane would not comment on yesterday's agreement until he sends a memo to the School Board, but officials said its cost would be close to the $32.4 million predicted when the superintendent unveiled his merit pay proposal last year.

Although unions have no bargaining power in Virginia, Spillane has said teacher support for the merit pay plan -- the first in the Washington area -- is crucial to its success. The FEA and school officials began negotiating Sept. 15 and the union had set Sunday as its deadline for reaching a new contract. Dash did not say what the union would have done had it failed to reach an agreement, but the FEA staged a work slowdown two years ago to express dissatisfaction with its contract offer.

The county's other union, the 900-member Fairfax County Federation of Teachers, is so fundamentally opposed to the merit pay plan that an agreement between the union and the school system is unlikely. Federation President Rick Nelson said yesterday that his union will wage an educational campaign against the pay program, which he contends gives too much power to school principals, and said the FEA won only "garbage" concessions.

Forty percent of the county's 7,600 teachers are being evaluated this year under the new system. Although merit raises will not be based on the results until 1989-90, teachers rated marginal will have their pay frozen next year and teachers rated unsatisfactory are to be fired. Dash said yesterday the FEA still plans to file a lawsuit to stop the system from freezing pay of marginal teachers.

All agree that teacher morale has deteriorated because of the new system, as it has across the country where other merit pay plans have been installed. School officials say the stress will subside as teachers ealize that decisions are being made fairly, but the FEA blames some of the slumping morale on problems with the evaluation system.

Dash said yesterday that morale problems also vary from school to school, and "people who feel it's not being implemented in their buildings are going to vote against" the pact.

If the contract is approved by the FEA, it would be subject to School Board approval as part of the budgeting process that culminates with review by the County Board of Supervisors in the spring.