Montgomery County's 6,500 school support workers have rebounded from a financial blow by the County Council and a rare move by the Board of Education to renegotiate their contract, winning the raises they had expected all along.

School bus drivers, secretaries and other workers will get pay increases of 6 percent next year and 5.5 percent in the two years after that under a new contract signed yesterday by Montgomery school officials and leaders of the employees' union.

The three-year contract is nearly identical to the one ratified in February, before the salaries of school employees became a casualty of severe cuts by the County Council in the school system's budget request. The result of an election-year taxpayers' revolt and growing competition for county money, the reductions have created an arduous spring for school officials and employees.

Yesterday, Vincent Foo, president of the Montgomery County Council of Supporting Services Employees, said, "We feel very pleased we were able to hold onto the original wages."

The agreement came 10 days after Montgomery school board members voted reluctantly to reopen contract talks with the workers' union and the Montgomery County Education Association, which represents the county's 7,200 teachers. It was only the second time in county history that the board chose to revise its labor agreements.

In the new negotiations with support workers, the school board initially offered 5.5 percent raises for the coming year but consented to the full 6 percent increase in exchange for a possible reduction in benefits for members who belong to health maintenance organizations. The union consented to renegotiate those benefits in a year.

Robert E. Shoenberg, the school board's president, said board members have not yet decided how to pare $2.3 million from other parts of its $703 million 1990-91 budget to give workers higher raises and benefits than the council intended.

Negotiations with teachers are proving difficult. According to sources, school officials are offering teachers a 5.5 percent raise for the coming year. The proposal is a slight improvement over the 5.2 percent raise that the council had envisioned.

But teachers' negotiators are pressing for the entire 7 percent raise furnished by the contract that they signed last winter.

The two sides appear likely to bring in an independent mediator next week, sources said yesterday.

Both the school board and the teachers are running out of time. The board must approve a final version of its budget before the new fiscal year begins July 1. And union President Mark Simon has said union leaders will not put a contract to a ratification vote, as the union's rules require, after the school year ends June 15.