The Fairfax County School Board unanimously approved a record $738.5 million budget request last night for the next school year after adding funds for replacing aging equipment and for teacher aide salary increases to the plan proposed by Superintendent Robert R. Spillane.

The funding blueprint for the area's largest school system now goes to the Board of Supervisors. Some county officials and supervisors have criticized the size of the budget, which would require a 16.9 percent rise in county general funds, the largest such increase since 1982. The budget would be nearly 10 percent higher than this year's spending.

But Anthony Lane, who heads the School Board's budget committee, said he is confident the supervisors will endorse the school proposal. "I've not heard any negative remarks at all," he said.

Included in the budget would be an average 8.5 percent salary increase for teachers -- the second of three years' worth of raises that were promised in return for teachers' agreement to the superintendent's merit pay plan. Spillane's budget also proposed an 8 percent pay raise for bus drivers and hefty raises for principals, who would receive salaries of up to $74,691, but only 4 percent increases for most other school employees.

Those lower-paid employees turned out in force at recent public hearings to protest their proposed raises. Heeding some of their pleas, the School Board added more money for teacher aides, whose starting salary is now the lowest in the Washington area and whose top pay is less than Montgomery County's starting salary.

Aides' $9,431 starting salary would be raised 6 percent, to $10,000, with other salary steps raised in tandem to a maximum of $15,255. The cost would be paid for by reducing the substitute teacher budget, salary increases for elementary school assistant principals' salaries and outlay for some new computers.

The board defeated an attempt to excise the raise for principals, but four board members criticized Spillane for what they described as springing the proposal on them with too little notice. "I almost feel it's a slam dunk strategy," Kohann Whitney said.

The other major issue in the budget hearings was lack of proposed spending for maintenance and new equipment at aging schools. Spillane had said there was not enough money because of cuts in state funding and the need to increase salaries.

The board reversed him and added $2.1 million for replacement equipment such as furniture and copying machines, but defeated a motion to include $1.5 million more for roof repairs.

The two most prominent new spending items in the budget are teacher raises costing $46.2 million and more than $8.6 million for the costs of growth due to new housing construction and a mini baby boom.