The Fairfax County School Board approved a $531.2 million operating budget request last night that would give all school employes a 4 percent raise and fund the Washington area's first high school for science and technology.

The budget for the 1985-86 school year, which will be forwarded to the County Board of Supervisors for a vote April 22, represents an increase of $23.8 million, or 4.7 percent, over current spending.

Still, the budget is not expected to raise county property taxes because of an increase in tax revenues from commercial and residential development.

School Superintendent William J. Burkholder's original budget proposal remained substantially intact after the board's action.

Under the proposal, the county's share of total school costs would increase over last year by $28.3 million (8.8 percent) to $349.5 million, the margin of growth set as a guideline by the supervisors.

"I think this budget focuses right in on the classrooms and right in on the students," said School Board Chairman Mary E. Collier.

The largest single item in the budget, included in $8 million worth of new programs, is $435,485 to hire 20 teachers for grades one through three in 19 schools as part of a system-wide effort to improve the performance of minority students.

The School Board voted 5 to 4 last night to remove a budget proposal for new school roofs and to substitute $1.12 million for the reduction of pupil-teacher ratios from 27 to 1 to 26 to 1 in grades four through six.

The budget also calls for $1.1 million for more school buses, supplies, textbooks and instructional staff for the Thomas Jefferson High School for Science and Technology, which is expected to open this fall and serve students from Fairfax, Prince William and Loudoun counties, as well as the cities of Manassas, Manassas Park and Falls Church.

Included in the budget is a $4.9 million proposal that would, in effect, give teachers more than the 4 percent raises. It would add three additional "snow" days to the 180-day school year mandated by state law.

Teachers say adding three days would solve a community problem of seldom knowning when the school year is going to end and provide teachers with an average 1.7 percent more pay. The budget also requests funding to hire 90 new teachers and administrators to handle a growing student population. Administrators expect Fairfax' student population to swell next year by 2,166, from 124,040 to 126,206.

The School Board agreed to ask the supervisors for $5.2 million in addition to the $531.2 budget request for such things as pupil-teacher ratios in some classes, career center coordinators and instructors in Japanese, which would only be funded if extra money can be found.