Fairfax County school officials said yesterday they will drop their plans to charge tuition for military dependents at Fort Belvoir because the U.S. Department of Education has promised that federal impact aid is on the way.

Fairfax School Board Chairman Ann P. Kahn said the county has been assured of $3.5 million in federal funds--$1.4 million less than the amount sought by the county but enough to rule out tuition fees for the current school year.

The agreement appears to end a five-month legal wrangle in which the Justice Department challenged the legality of the county's plans to charge tuition to military families in the face of federal budget cuts. The impact aid at issue reimburses localities for providing schooling to children of government personnel living on federal reservations that pay no property taxes.

Lawyers from Justice are expected to ask a U.S. District Court judge in Alexandria today to dismiss the federal government's suit involving Fort Belvoir on grounds the issue now is moot.

An assurance of aid for York County in the state's Tidewater area recently led to resolution of a similar Justice suit.

Kahn said yesterday Fairfax County officials are delighted that proposed tuition charges now appear unnecessary. "The school board has always felt that the cost of educating these children is the responsibility of the federal government and that the parents should not be the ones to assume these costs," she said.

Kahn cautioned, however, that "although the problem is resolved for this year, a permanent resolution still must be reached in view of proposed future cuts" in impact aid.

An Education Department official said Fairfax will receive half of the promised $3.5 million by mid-March from funds allocated to the department under a continuing budget resolution voted by Congress. The remainder will depend on future congressional action, he said.

First word of a break in the Fort Belvoir impasse came in a telephone conversation Feb. 17 between Vincent E. Reed, assistant secretary of education, and Fairfax School Superintendent Linton Deck, according to the Education Department.

A letter sent Monday by Deck to Reed confirming the county's intention to drop its tuition plans affecting about 1,500 children of servicemen stationed at Fort Belvoir was included in documents filed yesterday in federal court in Alexandria.

An official said similar promises of federal assistance have been issued to school officials in Norfolk and Virginia Beach, which have heavy concentrations of military personnel.