The first time the Fairfax County Board of Education tried to buck the Defense Department, the federal government kicked back with a lawsuit.

Now the board is trying another tactic. And this time, Uncle Sam's hands may be tied.

The school board will vote tonight on a proposal to oust Fort Belvoir from the county public school system and establish the Army base as a separate school district. The Fairfax board would be one of the first in the nation to use that maneuver to combat expected cutbacks in federal subsidies for educating military children.

The local and federal governments are jousting over who should pay the bill for educating the 2,000 children whose parents live and work at Fort Belvoir in southeastern Fairfax County. Although the controversy involves only about 1 percent of the students in Fairfax public schools, the outcome of the fight will set precedents for dozens of school systems nationwide trying to cope with cuts in the impact aid program.

The vote tonight will be the Fairfax school board's second attempt this fall to diffuse the impact of the federal budget cutbacks on the local school system.

The board originally threatened to impose stiff tuitions on the Fort Belvoir children unless Congress continued subsidizing the youngsters' education at last year's levels. President Reagan has asked Congress to approve massive budget cuts in the federal impact aid program, which is designed to compensate school systems for the costs of educating the children of certain transient federal employes, mainly military, who don't contribute to the tax base of the local districts. Congress is still haggling over the proposals.

The Virginia General Assembly earlier this year approved legislation allowing school districts to charge tuition to military children if federal impact aid payments dipped below 50 percent of the cost of educating the students.

Fairfax County was one of the first in the state to threaten to use the law.

In response, the Justice Department, on behalf of the Defense Department, filed suit against Virginia and Fairfax County, attacking the constitutionality of the proposed tuition charges.

But school officials think they have found legal refuge in the laws that give states the sole authority to draw school district boundaries. Although the proposal to oust Fort Belvoir from its existing district starts with the school board, it would face a long political route requiring approval of the county Board of Supervisors and the state Board of Education. The Virginia General Assembly would have final veto power over the proposal.

The school board already has started picking up support from members of some of those other governing authorities, however.

"We're going through all these maneuvers to force the Defense Department to do what they should be doing," said John Herrity, chairman of the county board of supervisors.

The Defense Department, say Herrity and school board members, should be responsible for financing the education of military children. Fairfax County taxpayers shouldn't be saddled with the costs of educating children whose families live on the base and don't pay county taxes and who aren't legal residents of Virginia, they argue.

They point to neighboring Quantico Marine Base, just southwest of Fort Belvoir, where the federal government operates the school system for the 1,500 students living on the base.

"On the bottom line," said Herrity, "all we want is equality."

But Quantico is different, argue federal officials. It is one of 23 military installations nationwide where the Defense Department operates the schools. In most of those areas, including Quantico, local schools are too small and too poor to support the large infusion of military students, according to federal authorities.

And that's just not the case in Fairfax County, federal officials charge. "Fairfax has a viable school system with the capacity to incorporate the military students into their schools," said one Defense Department official involved in the controversy. "The percentage of military students in the system is not that high."

Federal officials add privately that the Fairfax public school system -- one of the wealthiest in the nation -- can well afford to educate the 2,000 youngsters who live at Fort Belvoir. The total student enrollment in the county is about 124,000 for the 1981-82 school year.

The Fairfax County school board agreed in 1969 to take over the three elementary schools at Fort Belvoir and provide education for the secondary students in schools off the base.

According to federal officials monitoring the controversy, no school systems on military bases have been put under federal government control in recent years.

School board members have taken the microphone at school board meetings for the past six months to vent their personal outrage against Reagan's proposed cutbacks in impact aid. The cutbacks are nothing new, however. Every president since Dwight Eisenhower has tried to eliminate the program.

Fairfax school officials say they are fed up with the fight.

"It's become obvious that impact aid will continue to be cut until the whole program is cut out," said John Hess, director of financial services for the school system.

And while carving Fort Belvoir out of the school district would solve some headaches for the state and the county, it would create a few additional ones.

For Fairfax County, the most immediate problem would be absorbing the more than 100 teachers and other staff members at the three schools on the base and those off the base that accommodate the military youngsters. With student enrollments shrinking countywide, there is a corresponding decline in the need for teachers in many areas.

And while Fairfax County would save money under the plan, the proposal would end up costing the state and federal governments more money.

For the state of Virginia, financing Fort Belvoir as a separate school district would cost substantially more per student than financing the children's education through the Fairfax County school system. A large chunk of state aid is determined by the property values, income levels and sales tax receipts of local governments. The more that comes from these sources, the less the state contributes.

Because of the low property value, relatively low income levels and virtually nonexistent sales tax collected on the base, the state would be contributing more per pupil than it did when the Fort Belvoir students were lumped with the wealthier Fairfax County student membership.

And for the federal government, the cost of educating the students would escalate likewise. Under impact aid proposals now pending in Congress, the federal government would give Fairfax schools slightly less than half the total cost of educating the military students. But with Fort Belvoir as a separate district, it would have to pick up the total amount not covered by state contributions -- about two-thirds of the bill.

Admitted one federal official: "We get a good deal now, there's no doubt about that."