A federal judge gave the green light today to a magnet school plan for desegregating Prince George's County public schools, but he said the plan should be fully implemented in less than the proposed five years and include mandatory backup busing that would go into effect if the magnet concept fails.

U.S. District Court Judge Frank A. Kaufman's preliminary endorsement of the proposal means that the first phase of the magnet system can begin this fall, and it signals substantial movement in the 13-year-old desegregation suit.

"I sense more agreement in this case than I've seen since 1975," he said.

But Kaufman made clear that a final order on the case will come only after he sees how the details on the plan are worked out and how it is accepted by the community.

"I think the magnet proposal is one that should be fully explored," Kaufman said at the two-hour hearing. " . . . But I can't say that after everything is learned that the magnet plan will be the focus of this court."

The magnet system submitted by the County Board of Education to Kaufman in early May would establish 30 magnet schools over five years, with a first-year cost of $8.7 million. In the first year, there would be six schools for talented and gifted students and six others offering before- and after-school day care.

Administrators hope that magnet programs will attract students to schools outside their segregated neighborhoods, thereby integrating the system of 105,000 students and 175 schools.

Kaufman's actions today set aside, at least temporarily, the controversial recommendations of an expert panel, which had suggested more extensive busing and school closings. Kaufman said some of the recommendations of the panel, headed by Robert L. Green, president of the University of the District of Columbia, may be implemented later. But the judge cautioned that he wanted to keep travel time on buses shorter than those called for in the "Green Report."

For parents, the magnet plan means that they can apply to send their children to schools other than those to which they are now assigned. A second aspect of the plan provides extra funding for the 10 schools that officials have said will not meet the court-mandated requirement of no more than 80 percent and no less than 10 percent black enrollment. That will allow the classrooms to be less crowded and have more staff and computer equipment than other schools in the system.

Implementation of the magnet proposal will not alter the county's current busing plan, but it will entail additional voluntary busing for children accepted in the magnet schools for talented and gifted students. Parents who opt to send their children to other magnet schools offering before- and after-school day care will have to provide transportation.

Students will be accepted to the new programs according to racial guidelines, so the magnet schools will be integrated. And students will have to meet certain criteria to be accepted in the talented and gifted program.

Kaufman's actions drew generally favorable response from school officials, parent groups and the county NAACP.

"It's a very positive step forward," said Superintendent John A. Murphy, who drew up the magnet proposal. "I'm delighted the judge instructed us to go ahead."

State Del. Albert R. Wynn (D-Prince George's), a member of the Ad Hoc Committee on Quality Education, which has been critical of the magnet proposal, expressed some disapppointment.

"We were very strongly opposed to the TAG talented and gifted magnets," said Wynn, arguing that this program would benefit white children more than black children. "The board is in fact committed to TAG magnets. We would have liked to hear more discussion on that today."

Clement Martin, president of the county NAACP, said Kaufman's instructions were "favorable on several of the things we asked for." The civil rights organization, in a response filed with the court last week, offered general approval for the magnet proposal but suggested nine modifications, several of which Kaufman supported today.

Kaufman gave what he called a "strong tentative view" agreeing with the NAACP that a specific mandatory backup busing plan must be included in the plan. "We just don't have time to spare to try a voluntary plan and then if it fails to get a mandatory plan up," he said.

He agreed "tentatively" that the five-year time frame proposed for the plan should be shortened. And he supported the NAACP's argument for a more substantial compensatory program.

Kaufman instructed attorneys for both sides to hold work sessions beginning June 17 to iron out their differences.

Kaufman was firm in ordering the Board of Education and the county NAACP to improve cooperation and move ahead rapidly on a desegregation plan. The two sides have been involved in litigation since 1972, when the civil rights organization filed suit against the county schools.

"We've now been about our efforts to try to achieve unitary status [desegregation] for more than a decade . . . ," said Kaufman. "If we could have a little bit less fighting and scrapping about every little issue, we could move ahead."