The Prince George's County Board of Education voted last night to ask the County Council to increase school spending by $47 million to pay higher teacher salaries, expand the magnet school desegregation effort and buy new books and supplies in the next school year.

In addition to approving its $477.7 million budget request, the board voted to support the establishment of a residential math/science high school in the state, despite protests from parents and opposition from the union that represents the school system's 6,000 employees.

Prince George's County is a leading candidate to be the site of the school, a top priority of Gov. William Donald Schaefer, but a subject of criticism from some state legislators who say it is too costly and unnecessary.

The budget request, $1.8 million more than proposed by Superintendent John A. Murphy, would increase school staff by 355 positions to prepare for an expected increase in enrollment and to provide administrative help for some elementary school principals.

The request includes money to place new computers in second- and third-grade classrooms at all nonmagnet, noncompensatory schools.

The budget request represents the latest attempt by the board to reverse the effects of economies that resulted from a cap on property tax receipts imposed 10 years ago when voters approved TRIM (Tax Reform Initiative by Marylanders). TRIM, an amendment to the county charter, has since been modified to permit greater spending.

"We're trying to phase back in what we lost under TRIM," said board Vice Chairman Doris Eugene.

Last year the County Council cut $15 million from the schools' spending request. Council members predict a similar fate for this year's request, which represents more than 57 percent of the county's budget.

Superintendent Murphy said the residential math/science high school "would be a plus" for county students and urged the board members to support it.

But board members Marcy C. Canavan (District 9), Suzanne M. Plogman (District 2) and Catherine M. Burch (District 3) abstained, citing several concerns.

They noted, among other things, that the school would be run by a board of directors that would not be accountable to the state board of education or the Maryland Department of Education and that teachers at the school could be exempted from state certification requirements.

"What's good enough for us should be good enough for them," Canavan said.

The school board also announced that it has awarded 20 percent of the total value of its contracts to minority businesses. The percentage was double the board's goal when it established a minority business procurement program in 1986 and compares with 6.5 percent of total contracts awarded to minority businesses last year.

But John Spearmon, a spokesman for the Coalition for Black Economic Development, said the schools should do more. "We've pushed for set-asides of 30 percent," he said, referring to mandatory awards to minority firms.