The Prince George's County Board of Education last night adopted a sweeping expansion of its six-month-old magnet school plan, dismissing objections that it was moving too fast in implementing the ambitious desegregation effort.

The expansion, approved on an 8-to-1 vote, will accelerate the timetable originally proposed last year, establishing 20 new magnet programs by the 1987-88 school year. It will also create five new "compensatory education" schools, giving predominantly black schools additional resources because officials say they cannot be desegregated. Seven other schools will receive compensatory funding on a temporary basis, and five schools will be closed.

"No one has ever offered a desegregation proposal that offers as much educational opportunity to black children," said board member Thomas Hendershot. "We must act and we must act now, or give the job to someone else."

The proposals adopted last night constitute the second phase of the desegregation effort introduced this school year in an attempt to resolve an ongoing lawsuit against the county. There are now 12 magnet schools offering special programs as an incentive to white students to attend predominantly black schools.

The sole opponent to the plan was board member Barbara Martin, who complained that a proposal she had offered -- which would make Dodge Park Elementary a compensatory school -- had been tabled for more study.

Sarah Johnson said she voted for the expansion with great reservations, particularly about increasing the number of compensatory schools. "This is a plan that segregates rather than integrates," she said. Johnson and Martin are the board's only black members.

The proposed second phase, offered by Superintendent John A. Murphy last month, has recently drawn criticism from the county NAACP, a plaintiff in the 1972 lawsuit, and the County Council. Both groups argued that the board was moving too fast and needed to evaluate the program more thoroughly.

"There are too many unanswered questions and this board should not put forward any new program," said Richard (Steve) Brown, executive secretary of the county NAACP.

The board, bowing to pressure from angry parents, abandoned plans contained in Murphy's proposal to close Forestville High School. Instead, the school will temporarily receive additional funding and eventually offer magnet programs.

The board also modified Murphy's proposal by:

*Creating two new programs for talented and gifted students to accommodate black students who are on waiting lists for similar programs at two other schools.

*Instructing Murphy to create one or two new before- and after-school day care programs to accommodate black children who have been unable to enter six existing "extended day" schools because of lack of space and racial quotas.

*Voting to keep Benjamin Stoddert Middle School open as a middle school for grades six through eight. It was designated to become an elementary school.

*Creating a "laboratory" school at Northwestern High in connection with the University of Maryland, through which university staff will work with high school students.

The new magnet schools will offer programs including back-to-basics curriculum, foreign language immersion and performing arts.

Voting for the proposal, in addition to Johnson, were Chairman Paul Shelby, Vice Chairwoman Lesley Kreimer and members Doris Eugene, Catherine Burch, Angelo Castelli, Norman Saunders and Hendershot.