The nest of a couple of common black mice is an insignificant thing, a flimsy construction of twigs and debris no bigger than a kitchen scouring pad.

But just such a nest, built on a Fairfax County school bus by mice that authorities suspect were left there by teen-age pranksters, will end up costing county taxpayers more than $7,000. That's how much will be required to fix the bus, which will be out of service for the rest of the school year.

Bus driver Ruth Zick was heading through the Baileys Crossroads area on her way to pick up students bound for J.E.B. Stuart High School Monday morning when she turned the vehicle's front window defroster, according to school officials.

Those who reconstructed the incident later said that what happened next was a direct result of homebuilding by mice in the defroster's fan.

Remains of a mouse next sprayed from the defroster and hit Zick in the eye.

"It blew fuzz, hair and debris all over everything," said Joseph P. Higgins, director of transportation for county schools. "It would alarm anybody to be driving down the road and have something like this fly right in your eye."

The bus went into the ditch near the corner of Magnolia Lane and Mary Alice Place, Zick, her daughter and granddaughter, the sole occupants, were uninjured. The bus has a broken front axle, a bent frame and a dented fender. It won't be the same again for months.

It was quickly recollected that last Friday Zick was forced to pause in her morning bus run in the same neighborhood when some of her student passengers started shrieking and leaping up on their seats. The passengers were shifted to another vehicle and Zick's bus was searched.

Twenty-five black mice were found and removed on that occasion. After Monday's accident, three more were found -- perhaps the nest builders.

"Of course, we don't know where the mice came from," said Charles W. Clements, a school transportation supervisor.

But suspicion lies with the passengers.

Any student found responsible will face severe disciplinary action, declared Stuart High Principal Richard Johnson.

"This isn't just a prank situation," Johnson said. "Any time that children's lives are in danger, it's a very serious thing."

But the incident was not without redeeming value. "I was afraid of mice before this, but not any more," said Zick.