After an emotional meeting attended by almost 350 persons, the Arlington school board last night informally agreed to adopt a proposed $59.8 million fiscal 1982 school budget.

The board's action, scheduled to be formalized next Thursday, would be in defiance of an order by the County Board's Republican majority to pare the county's proposed share of the school budget from $46.8 million to $43.5 million.

The County Board had issued "guidelines" to the school board setting a maximum increase of 7 percent over the current $40.7 million county share. But superintendent Larry Cuban -- who has since left office -- proposed a budget calling for a 15 percent increase to compensate for inflation. No new programs were added.

To a standing ovation from parents, teachers and students, Republican-appointed school board member Evelyn Reid Syphax moved to adopt Cuban's proposed budget untouched. She was supported by Democrats Torill G. Floyd and Ann C. Broder, assuring a school board majority when the final vote is taken.

Arguing that the County Board is unlikely to agree to contribute its designated share of the school budget, Hilton predicted that the school board may eventually have to make cuts anyway.