Fairfax County School Superintendent Robert R. Spillane last night urged the School Board to approve a three-year plan to spend millions of dollars for more computers in the classrooms, a proposal that some board members and parents are questioning.

The proposal would allot $2.9 million in 1986-87 and an unknown amount in the two years after that to acquire one computer for every 20 elementary school students, 18 intermediate school students and 13 high school students. The system now has one for every 118 elementary students, 150 intermediate school students and 78 high school students.

Some School Board members have raised doubts about the plan, questioning whether the schools are moving ahead too quickly without an assessment of progress so far.

And the County Council of PTA's computer committee voted last month to urge the board to spend no more than $800,000 on computers for general education in 1986-87 -- half of what Spillane requested.

Mary Anne Lecos, assistant superintendent for instruction, told the board that the proposal was scaled back from original plans to meet objections of critics who said it was going too far too fast.

Board member Laura I. McDowall said she was pleased the plan had been reduced in scope, but member Joy Korologos said she still had questions about "expanding a program before we evaluate it."

Michael L. Hays, chairman of the PTA computer committee, said yesterday the panel "is generally dismayed by this plan. We are concerned that because of the hurry-up way they're doing it there will be a great deal of waste and inefficiency."

Hays said the superintendent's staff has offered few details on how the plan would work, and no proof that the high-tech effort would produce a better education than at present. He said the committee had serious questions about the ability of the schools' central office to manage the computer program.

Hays said the committee's attitude is that the schools should slow down and assess what they are doing before plunging ahead.

"I have a general sense that parental pressure of a few years ago for computers in the schools has very much abated," he said.

Under the plan, by the 1988-89 school year each elementary school student would be provided 1.3 hours a week of computer time, intermediate students 1.4 hours and high school students 2 hours.

In the regular classroom, the aim would be to move from the current focus on teaching computer operation to using them to teach writing, problem-solving and science.

The three-year "instructional technology plan" also would expand use of computers for special education students, vocational and business classes and gifted students.

In other business, Spillane told the board that County Executive J. Hamilton Lambert said he will recommend to the County Board of Supervisors appropriation of $2.6 million to replace 77 school buses dating to 1973 and 1974. Those 77 buses had been included in Spillane's proposed "wish list" to be funded if more money was available beyond his proposed $589 million budget for 1986-87.

That budget calls for 50 new school buses to accommodate enrollment growth and 33 buses to replaces aging vehicles now on the road. Even if the supervisors approve the 77 replacement buses, the county schools will still have 250 buses on the road that were built before new federal safety standards took effect in 1978.

Several parents of Cooper Intermediate School children injured in an accident last October involving a 1973 bus urged the board to replace those older buses quickly, saying the injuries might have been less severe in a newer vehicle.