A lawsuit seeking to halt across-the-board drug testing of Fairfax County school bus drivers on the grounds that it is an unconstitutional invasion of privacy was filed yesterday in U.S. District Court in Alexandria.

The complainants -- 16 drivers, the American Federation of State, County and Municipal Employees and the county branch of the NAACP -- also asked for a preliminary injunction to stop the testing immediately.

The lawsuit charged that in addition to violating bus drivers' rights of privacy from unreasonable search and seizure, the tests are unnecessary because there is no evidence of widespread drug use among drivers. The School Board, which approved the drug tests in June, said they are necessary to assure the safety of the 97,000 county children who ride Fairfax school buses every day.

Ned Carr, an assistant school superintendent, said yesterday the county is confident it will win the lawsuit and has no plans to drop the drug testing, although it will investigate drivers' complaints of poor medical care by the clinic that performs the drug tests.

The county, which operates the nation's largest school bus fleet, began mandatory urine testing of its 1,002 bus drivers and permanent substitutes Aug. 1. Although it was not the first local jurisdiction to do so -- the District began drug tests in 1984 -- local unions vowed to make the Fairfax program a major test case.

Of the 512 drivers examined through Oct. 5, three tested positive for drugs, school system officials said yesterday. The three -- one who tested positive for marijuana, one for cocaine and a third for an unspecified prescription drug that affects driving -- are no longer driving buses, they said.

An additional 12 to 14 drivers tested positive for drugs on a first test, but upon taking a second test -- mandatory for those who test positive the first time -- came up negative, school spokeswoman Dolores Bohen said.

The drivers do not object to tests of drivers suspected of drug use, the lawsuit said. Some across-the-board testing may even be allowable -- but the Fairfax program is "unusually intrusive and inefficient," it said.

Some of the 16 driver plaintiffs also voiced complaints in the lawsuit about the quality of care at two Prime Care clinics, which perform the tests as part of the drivers' required annual physical.

In a similar case, an out-of-court settlement was announced Tuesday in which $315,000 will be split among 18 former District public school employes who sued the school board last spring after they were fired for alleged drug use. The lawsuit contended the tests were unfair and inaccurate.