School closings and mergers in Fairfax County, including the conversion of Fort Hunt High into an intermediate school, are expected to save the county $733,000 next year, according to adjustments to its $531.2 million education budget.

School Board members, who were given the revised figures at a committee meeting Thursday night, are expected to approve the merger and closing costs at the regular meeting of the board next week. The board approved its budget Feb. 20.

The board's finance and budget committee also was told that school lunch prices apparently will not go up next year unless federal aid is reduced, as President Reagan has requested.

The closing of Fort Hunt as a high school will save just over $1 million and eliminate 46.5 jobs, according to the figures. The closings of Bryant and Whitman Intermediate schools will save just over $1 million and eliminate 32.5 jobs. Opening Sandburg Intermediate School, formerly Fort Hunt, will cost $504,735 and provide 22 jobs.

The budget revisions, which include $469,800 in new county, state and federal money, also show:

* A 14.3 percent salary increase for bus drivers, costing $1.5 million, to bring their wages up to those paid by other area school systems and alleviate a driver shortage. The increase comes on top of a 4 percent increase for all school employes. It will bring a driver's starting salary, now $5.92 an hour, to $7 an hour. * Expansion of the Head Start program by five additional classes with 90 new students, paid for with new money from the Board of Supervisors.

* A three-year plan to place all elementary school principals and secretaries on a 12-month contract. Most now have 11-month contracts. Forty principals a year will convert to 12-month contracts, based on seniority. The cost will be slight in the coming school year, but is estimated at $241,725 the following year.