The Fairfax County School Board last night ordered its 1,100 school bus drivers to undergo urine tests for drug use before starting work next fall.

The plan for the drug tests, in which the drivers will give urine samples in private surroundings at a clinic, has brought threats of a lawsuit by union officials, who argue the tests would be illegal, inaccurate and an invasion of privacy.

Fairfax County, with the largest school bus fleet in the nation, will not be the first locality in this area to impose drug tests. The District of Columbia began testing school bus drivers three years ago. But officials of the American Federation of State, County and Municipal Employees said they regard the Fairfax plan as a test case.

Last night's 9-to-0 vote, with member Olivia Michener absent, followed a debate in which supporters argued that the drug testing is vital to assure the safety of the 97,000 county children who ride buses every day.

"It behooves us to be as careful as possible with who gets behind the wheel of a bus," said Chairman Mary E. Collier. Bus drivers already are required to supply urine samples for diagnostic purposes, she said. Urine tests can determine whether a person has diabetes. "It's less costly than one severe accident would be," said School Board member Anthony Lane.

The cost of the drug test will double the school system's current $30-per-driver expense for the required annual physical. One reason for that is the high number of false positive initial tests that will have to be confirmed or voided by costly subsequent tests.

Board member Anthony Cardinale, saying the school system can overcome problems of false positive tests and confidentiality, said the drug tests are "another safety factor."

Citing a union threat of a lawsuit, Cardinale declared, "I'm willing to take that risk."

Although lawsuits across the country have challenged drug test requirements for public employes, supporters of the Fairfax proposal argue that bus drivers are public safety workers subject to stricter standards than others.

But even some members who voted for the proposal expressed doubts. Kohann H. Whitney said she is not convinced there is a drug problem among bus drivers, and suggested the drug test proposal resembled trying to "kill a gnat with a steamroller." School officials have said there were no serious drug problems found among drivers this year.

"That's what this will show -- whether we do have a problem," replied Superintendent Robert R. Spillane. If there are few positive drug test results, the board could drop the tests in several years, he said.

Spillane said bus driving is changing from a job held primarily by women with small children who desire part-time work into a full-time occupation with a growing turnover factor -- 30 percent or more a year.

Representatives of AFSCME argued that the school system could achieve the same results with better supervision of drivers coupled with required drug tests when there is a reason to suspect a driver.

"Anybody who has a problem you're going to miss," said James August of the union's research office, saying that any drug-using drivers would abstain long enough to pass.