The Fairfax County School Board has declared war on the federal government. But the first shot wasn't fired at the adversaries -- the Defense Department, the Congress, Ronald Reagan. Instead, the first round hit the families of 1,520 military children living at Fort Belvoir.

Last week, the school board mailed letters to those parents warning them that they could be billed for the free public education their children have been receiving from the Fairfax schools -- unless the federal government coughs up enough money to cover at least half the costs of educating the children who live at the massive Army post on the southeastern edge of the county.

The school board has provided the parents with one out: Parents of children who live on the base would be exempt from the tuition if they register as Virginia residents. The tuition would not be charged to military personnel who don't reside on the base.

Even though the letter begins with an apology -- "We deeply regret that we must send you this letter" -- Fort Belvoir parents were angry that the board would even send out such a warning.

At a base where the average family's income is $11,322 a year, the proposed tuition charges seemed enormous: for an elementary school child, $2,600 a year; intermediate, $2,610; high school, $3,170. And for a handicapped child the tuition could be as high as $16,330.

The Defense Department has assured the parents they won't have to pay the bills, and even school board members concede they don't expect the families will have to pay the bills.

Despite that apparent agreement, however, both sides are continuing with their battle. The Defense Department says it will take the school board to court to fight the proposed action, while the school board hopes the mere threat of tuition bills will force federal authorities to continue funding for the military children.

But try explaining those war games to parents who found the letters in their mailboxes last week or read the reprinted copy on the front page of the Fort Belvoir newspaper.

At the base commissary last week, mothers gathered at the school supplies counter could only express bitterness, and some panic. While they were trying to save pennies on discount notebooks and crayons, the school board was threatening to charge them thousands of dollars for educating their children.

"We just can't afford it," said Linda Mitchell, whose husband is a sergeant. "My family will move back to Missouri if the school board charges us tuition. The plane fares to fly us home would be less than they plan to charge us in tuition."

The bill for the Mitchells' two elementary school-aged daughters would be $5,200 a year.

Some school board members said they hoped the letters would anger the parents. In fact, one of the main reasons for mailing the letters was to prompt parents to complain to the Defense Department and to exert pressure on Congress.

Even as they voted to send the letters, school board members conceded they did so with an uneasy conscience.

"The children shouldn't be held hostage for us to have a balanced budget," school board Chairman Ann Kahn said in an interview last week.

And another board member, James W. Kitchin, said he hoped the threat of tuition bills would build "a fire under somebody -- quick."

The controversy can be summed up in a word: money. Fairfax County is one of about 4,300 school districts that receive money under the federal impact aid program -- a program designed to compensate school districts that serve large numbers of students whose parents do not pay local property taxes because they live and work on federal property. The program includes not only military children, but students living on Indian reservations, in low-income housing projects, or on any federal property exempt from local real estate taxes.

Fairfax school officials estimate it costs the schools about $4.5 million to educate the 1,520 Fort Belvoir youngsters; the county now is scheduled to receive $2.6 million in impact aid.

But when Congress, which reconvened this week, begins its final financial considerations, Fairfax County officials fear their slice of the impact aid pie could be cut.

Although Congress already has decided to slash the impact aid program by 37 percent for the coming fiscal year, it hasn't decided exactly how to divvy up what's left. One proposal suggests cutting all impact aid to school districts unless 20 percent of their student enrollment lives on a federal installation.

Under that plan, Fairfax would lose all its impact aid, since the 1,520 Fort Belvoir children total just over 1 percent of the 125,000 students in county schools.

"The taxpayers of Fairfax County shouldn't have to bear the burden of paying for those children," said Kahn.

In anticipation of the financial cuts, the Virginia Assembly this year approved legislation authorizing local school systems to charge tuition to families like those at Fort Belvoir if impact aid dips below 50 percent of the cost of educating the youngsters. Since then, the North Carolina and Texas legislatures have passed similar laws, and more states are expected to tackle the issue in the coming year, according to James Maza, a lobbyist for an association of impact aid school districts.

If the federal government doesn't continue the funding and if court action protects parents from paying the tuition bills, the Fairfax County school board will consider closing the three elementary schools it operates at Fort Belvoir and barring the remaining children from attending the nearby county intermediate-high school, Kahn said.

The next move is up to Congress, which is expected to divide the impact aid fund sometime in October. Most school districts, including Fairfax, are waiting for the outcome before they take their next steps.

"It's not as though Fairfax County or the other local governments are being greedy about this thing," said Maza. "We consider this money a reimbursement for services rendered. If the federal government doesn't continue the money, they are welshing on a debt. And like creditors anywhere, if the bill's aren't paid, you pull out the services."

Try explaining that to the mothers carting out the bags of school supplies at the Fort Belvoir commissary.