The hood of the school bus is cracked open like a giant yellow clamshell, and Alysia Saddler leans in with a pointer and ticks through the checklist.

"This is the radiator cap. It can't be cracked or leaking or hot. The sight glass--when there's enough coolant, it's green. Hoses aren't supposed to have bubbles. There's the air compressor, the power steering is secure, the air lines, the steering shaft . . .

"The suspension, the spring hangers, the leaves, clamp, the U bolt is in the proper position, the drag link, the tie rod . . . "

It's an impressive display from a 24-year-old who weeks ago, before starting Howard County's school bus driver training, could barely identify the dipstick under a car's hood. Back then she was just a tow-truck dispatcher from Catonsville; now she's a commodity as rare as a moon rock.

With the opening bells of schools ready to sound, officials are struggling to get enough bus drivers behind the wheel. During prosperous times--Washington area unemployment was only 2.9 percent in June--any job is tough to fill. But it's hardest to get people to take a job that rarely guarantees full-time hours and only sometimes carries benefits. Add to that the perception that children's behavior and traffic have degenerated, and the entire nation is facing a driver shortage.

"I haven't heard from one state that says, 'I've got all the drivers I want,' " said Charles Gauthier, executive director of the National Association for State Directors of Pupil Transportation.

Fairfax County and the District still need 50 full-time drivers, and Montgomery County is looking for 30 or 40. Arlington County needs 20, and Prince William and Howard counties need 25 each. Even counties that are flush with full-timers, such as Prince George's, have nowhere near enough substitutes. And sometimes when officials think they have it under control, up bubbles bad luck.

"Two weeks ago, I thought we were in great shape," lamented Michael Lunsford, Loudoun County's transportation director. "Then we lost 10 drivers in the last 10 days."

School administrators and contractors--used by counties that don't run buses directly--are beseeching potential drivers by tacking up posters in churches, pounding signs into front lawns and tying banners to buses parked in shopping mall lots. Fliers that scream "Job Opportunity!" are tucked into Valu-Pak mailings, and ads creep across the bottom of the screen on local cable channels.

As David Drown, Howard's transportation supervisor, put it: "We're hoping to have people walk in off the street and say they want to be a bus driver."

But even if they do--even if dozens of people decide today to become school bus drivers--they will not be behind the wheel when school starts.

Before a trainee can get a learner's permit, schools check for drug and alcohol use, medical conditions, blemishes on driving records and criminal history. Then there's behind-the-wheel and classroom training. In the classes, which take a week to a month, drivers learn how to manage the 40 feet and 10 tons of yellow steel and the youngsters inside. Some school districts pay trainees; some don't.

Finally, applicants must get commercial driver's licenses from their states. It can take weeks to get an appointment for a road test, and drivers must memorize a vast inventory of facts.

"After all that," St. Mary's County transportation supervisor Sal Raspa cracked, "then you're gonna make a big 10 dollars and 32 cents an hour."

The starting hourly wage of $10 to $13 is comparable to a Metrobus driver's. United Parcel Service drivers start at $15.60 an hour; after two years they make $22.35 (a figure reached by few school bus drivers), and their cargo never sasses them. School bus drivers typically are guaranteed only five or six hours of work a day for 180 days a year, and some don't get full benefits.

As a result, many people take their training and drive a dump truck rich with overtime potential, an air-conditioned commuter bus, a delivery van. Or, after years of driving, they may leave altogether.

"We're losing drivers right to our own school system," said Lunsford, whose has seen employees move from buses into Loudoun's school buildings, as secretaries or classroom aides.

That's not to say that there's a dearth of people who love children and love to drive them, administrators say. They emphasize that the buses are filled with qualified, dedicated drivers and that standards remain high. But the schools are simply growing so fast that they can't get enough drivers.

In Loudoun, for example, the driving staff used to be packed with farmers. Now expensive new subdivisions sprout where crops once grew, filled with children who need to get to school but with few grown-ups who want $11-an-hour work.

Prince George's transportation director Kenneth Savoid anticipates resorting to the same backup plan he used last year: On desperate days, office clerks, technicians and supervisors--"the guys who wear white shirts and ties and whatnot"--got behind the wheel. At least they are licensed, he said, and "never have lost the touch."

Savoid relies in large part on substitute drivers who work full-time hours. This year, though, he's 100 subs short and has nothing to attract them with--not free training, not money to subsidize the $100 it costs to get licensed, not a competitive wage (they start at $10.03), not benefits. "It's a wonder we get anybody," he said.

Then there's the overwhelming duty of being accountable for as many as 50 children at a time. Russ Defibaugh, a bus contractor in Laurel, has tried to persuade several of his employees who work as attendants on buses for special education students to take the wheel instead. "All of them say the exact same thing: They have no desire to take on that responsibility."

Given the grim scenario, what are the schools doing to lure--and keep--drivers?

Fairfax County, which earlier this month was facing the possibility of staggering school start times by as much as 40 minutes, began offering $500 bonuses to new drivers and the employees who recruited them. The county is still short and might have to shift start times slightly--a remedy being considered in other districts, too.

Prince William changed some schedules, and many drivers who normally run three round-trips a day--one for high school, one for middle school and one for elementary school--will have to drive an extra elementary route.

And for the first time, Calvert County is giving drivers sick leave. Officials pray none of them takes it, though, said transportation director Brian Stevens: "We'll probably never have enough substitutes."

CAPTION: During bus driver training at Hammond High School in Columbia, Alysia Saddler, right, checks inside the emergency exit while fellow new drivers Chris Reddick, left, and Vernon Johnson, rear, and instructor Joyce Streett observe. Howard and other counties still need drivers.