The Prince George's County Board of Education voted unanimously last night to cut $10.6 million in improvements, including reduced class size and addition of dozens of staff positions, from its budget for the next school year to cover the cost of salary increases.

Board members said the cuts would severely limit their efforts to improve education in the $389 million spending program proposed for the fiscal year that starts July 1. Among the positions eliminated were reading teachers, guidance counselors and secretaries.

"It breaks your heart to see these things happen," said board member Thomas Hendershot. "The message that has to come out of here loud and clear is that the job ain't done yet."

The budget must still go before the County Council for action and will be returned to the school board for final approval before July 1.

County teachers recently ratified a contract that will raise starting pay from $15,738 to $19,000 annually and give other teacher raises averaging 6 percent. Custodians, secretaries, bus drivers and other "classified" employes recently received raises ranging from 5 to 6 percent. The pay package will cost $18 million.

The board had budgeted $8 million for salary increases, but to balance the budget was forced to eliminate the $10.6 million in improvements.

Among the cuts was $1.5 million to reduce class size, which now averages 28 students county-wide. The board initially planned to reduce by one the class size in each elementary school. As a result of the budget cuts, class size will remain at its present level, except in first grade, where it will be reduced by three.

Class size will be reduced to 20 in several "compensatory education" schools, which receive special resources because officials say they cannot be desegregated.

Board member Sarah Johnson expressed concern that the county was disproportionately funding improvements at the compensatory and magnet schools, which offer special programs to draw white students to predominantly black schools. "We're giving really good service to a few students," she said, "but I do think the majority of our students are being shortchanged."

Also cut was nearly $300,000 for new textbooks, $222,000 for new guidance counselors, $94,000 for in-school suspension centers and $224,000 to expand Project Success, a program for students performing below their ability.

The program is now in three high schools and the board had planned to expand it to four more, but will now move into only two more. Also cut were funds to place a full-time reading teacher in each elementary school and $13 million to buy new equipment, including school buses for the handicapped.

In approving the cuts, board members adopted a motion urging Superintendent John A. Murphy to "fight like hell" for more money from county officials. The fiscal 1987 budget reflects a $40 million increase over the current budget.