Members of a union representing nonteaching employes of the Prince George's County school system voted last night to postpone raises and take a three-day furlough next year in order to prevent 150 layoffs and 105 demotions caused by school budget cuts.

Earlier this week, the county teachers union rejected such a proposal. School officials have informed 827 employes--431 of them teachers--that they will be laid off because of the budget cuts.

Last night Jim Shearer, executive director of Local 2250 of ACE/AFSCME, told about 700 union members, "If you adopt this plan everybody in the union who is affected will get their jobs back. It is a decision you should make based on your own conscience." The union represents about 3,500 clerical, transportation, custodial and cafeteria workers as well as instructional aides.

The modification of the workers' contract will postpone until next April a 5.59 percent raise set for July 1 and an 18-cent hourly raise set for January.

Shearer told the meeting at Eleanor Roosevelt High School in Greenbelt that the union had an understanding with County Council members Gerard T. McDonough and Parris N. Glendening as well as the school board that should the county executive to be elected in the fall find additional revenues, they will be applied to restore the $1.9 million giveback by the union, which affects all the union's members except cafeteria workers.

Shearer told the crowd he expects Glendening, a Democrat, to be the next executive. The union has maintained that the incumbent, Republican Lawrence J. Hogan, has consistently underestimated revenues, thus worsening the budget squeeze.

Several union members expressed distrust at the assurances of politicians, but the vote was 3 to 1 to save the jobs.

"A lot of people here can't get another job; they sweep floors and clean bathrooms," said Tracy Papp, a cafeteria manager at James Madison Junior High School, adding: "Where are they going to get another job?" she added.

Earlier yesterday, about 200 parents and schoolchildren marched around the county administration building in Upper Marlboro to protest the school funding cuts that will force the layoffs.

The marchers, most pleading for favorite teachers from University Park, Riverdale and Magnolia elementary schools, circled the building waving makeshift signs.

"We just felt that something had to be done, some response had to be made before it's too late," said Jeanne Washburn, University Park PTA president.

The school board was given $31 million less than it requested to run the schools next year because of budget constraints forced by TRIM, the county's voter-approved limit on property taxation. County Executive Hogan has insisted that layoffs are unnecessary, particularly if workers renegotiate raises, which Hogan maintains are unnecessary.

Among the teaching employes being dismissed are all elementary school music teachers, advocates of whom dominated yesterday's march, bearing signs saying "Keep instrumental Music" and "The Fault Lies at Hogan's Door."

"It won't be anywhere near as fun at school" without music, said Craig Bricker, a 9-year-old pupil at Bond Mill Elementary School.

Most of the marchers had no idea what they wanted county officials to do about the budget shortage, beyond insisting that additional money for schools can be found somewhere. Many said they weren't sure whether teachers should sacrifice salary increases to save jobs. Many also admitted to having voted for Hogan and TRIM; both won by landslide proportions in 1978. "I was completely misled about what TRIM would do," said Cynthia Kelley of Mount Rainier, who added, "I voted for Hogan and I could just die."

A few council aides were on hand to receive the protesters' complaints. Hogan was out of the building and the council was not in session.

Several of the protesters sought to blame busing and racial change as causes of the school system's woes, and others accused the school officials of favoritism in making the cuts.

"I know they have all these people moving in from the District of Columbia and they have to pay for all the welfare," said Mary Dean Floyd, a librarian at University Park elementary who just received her RIF notice. "I just think they have to get their priorities straight. Education should come first," she said.