Fairfax County school officials, facing a school bus driver shortage so severe that one assistant superintendent called it the schools' number one problem, are intensifying recruiting efforts for drivers.

Frank Dixon, acting director of the school system's transportation department, said the school system is about 100 drivers short of a smoothly operating bus schedule. Dixon said the county's bus system, which is the largest publicly owned school bus fleet in the nation with 983 vehicles, tries to compensate for the driver shortages by transporting students from different schools on the same bus.

"We basically have to tear the bus runs apart," Dixon said. "It's the only way we can get them to school and get them home while we have this shortage . . . . Parents get very disturbed in the areas where we have runs doubled up. They just don't understand the problem."

Max Skidmore, an assistant school superintendent, said the bus driver shortage is most severe in the northern Fairfax County area, which includes Langley, McLean, Herndon, Great Falls and Reston.

Skidmore attributed the low supply of drivers from those areas to a higher median income level there than in other parts of the county. "The parents from there are seeking employment of a different kind," Skidmore said.

He said the bus driver shortage continues to be the school system's "number one problem" with high daily absenteeism and a consistent monthly turnover rate.

School Board Chairman Mary E. Collier, calling the county's school transportation system "really stretched," said she was worried whether there would be enough bus drivers to operate the 50 additional buses ordered for next school year and included in the School Board's proposed $586 million fiscal 1987 budget.

Collier said the supply of bus drivers increased slightly last summer, but only after board members agreed to raise the starting hourly salary to $7 from $5.07 and to increase the health and life insurance benefits for full-time drivers.

"We also allow mothers to take their preschool-aged children on the bus with them," Collier said. "That helps because there are some mothers with children who didn't want to take a job and leave their younsters elsewhere ."

In the most extensive campaign ever to get new drivers, Dixon said school buses have been displayed in area shopping center parking lots each Saturday to advertise the bus driver job openings.

Other recruiting methods have included advertisements in newspapers and school publications, speeches by school administration officials and letters to area colleges and service organizations.

"It's just a revolving door. A lot that leave us have worked a month and then quit. We have a continuing need for more drivers ," said Skidmore.

A majority of the current bus driver pool are women, and school officials said they have turned recruiting efforts toward attracting more retired persons under 70 years of age.

Persons between the ages of 21 and 70 are eligible to apply to become a school bus driver, but they must have a valid Virginia driver and chauffeur license and an "impeccable" driving record for at least the past five years. Dixon said persons 18 years old could be hired for the job, but only with Superintendent Robert R. Spillane's approval.

R. Warren Eisenhower, an assistant school superintendent, said that about 30 percent of the district's 809 full-time bus drivers live outside Fairfax County. He said the drivers are paid biweekly and usually work between 35 and 40 hours a week, with overtime pay available for field trips.