The Arlington school board unanimously approved last night a budget calling for a county contribution of $48.5 million in the next school year and providing for 5 percent raises for all school employes.

Under the proposal, which now goes to the County Board, 24 nonclassroom jobs, including four top administrative posts, would be cut.

The budget proposal is $1 million more than the County Board has set as a school spending guideline for next year and represents a $3.8 million increase over this year's county contribution to the schools.

In addition to the local funds, the schools are expecting $13.5 million in state and federal aid, raising the total school spending proposed for next fiscal year to $62 million.

At the board's request, new Superintendent Charles E. Nunley had prepared four possible spending packages ranging from $47 million to $50 million in county funds. He recommended a $49 million local contribution, but said it would cost $48.4 million in local funds next year to run the schools at this year's level.

Board member Simone (Sim) J. Pace proposed a $48.3 million county contribution last night, with the backing of members Claude M. Hilton and Michael E. Brunner. But board members Torill B. Floyd and Evelyn Reid Syphax prevailed upon them to add another $200,000, including $52,500 for kindergarten aides.

Pace said that while his proposal cut deeply into administrative funds, it had many instructional features that would have been included at the $50 million level.

Among the improvements are the addition of an English teacher at each high school, new part-time teachers to coordinate the computer education program and to strengthen the elementary science program.