After watching her big brother and sister ride off to school every morning for five years, Christina Trimble was ready for her turn on Tuesday.

She was wearing her ruffled jean skirt, a glittery butterfly T-shirt and slightly crimped hair from an overnight experiment with curlers. With her pink lunch bag and rolling purple backpack stocked, she knelt in the front hall of her home in Arlington County, petting her dog Sam while the kitchen clock ticked off the minutes.

A little before 8:30, her family huddled on their front porch to snap pictures. Then her sister, Audrey, announced with a third-grader’s confidence, “That’s the squeak of our bus.”

Sure enough, the yellow bus rounded the corner — all 38 feet of it — and screeched to a stop in front of their house. The doors opened, and Christina darted off the porch, dragging her backpack down the walk. Her mom, Alina Trimble, caught up and gave her a quick kiss before she scrambled up the stairs ahead of her sister.

The first step in an academic career is often a bumpy ride on smooth, green seats. Before too long, those 20- or 40-minute rides will fade into a lifetime of commuting and traffic jams. But for at least one day, for a 5-year-old, it can be a jittery moment of separation and expectation, of exhilaration or utter terror.

Bus drivers learn to deal with the needs and nerves of their youngest riders. They memorize names, smile reassuringly and watch for mood swings.

“If they have a good day on the bus, they are probably going to have a good day in school,” said Gregory Sutton, transportation director for Arlington schools.

Moving from the family minivan to a big bus involves no small amount of retraining. The riders don’t have to buckle up, but they do have to sit forward with bags on laps and STAY SEATED.

Seasoned drivers have various tricks for handling a panicked kindergartner on the first day. They might break the ice with a compliment or a joke. They might enlist help from a parent or a fifth-grade safety patrol member to cajole the child into the seat.

Chris Schrader, a father in Chevy Chase, said getting on the bus was a “gut-wrenching experience” for his daughter Grace when she started kindergarten last year.

“She would tear up at night thinking about it, and then tear up in the morning,” he said.

The sometimes nervous but normally well-behaved little girl liked kindergarten and loved her teacher at Rosemary Hills Primary School in Silver Spring. But she did not love the bus.

One morning, Schrader recalled, Grace faked out his wife at the bus stop and sprinted halfway home. Another, she fought so much he drove her to school. “I had to give her the silent treatment in the car,” Schrader said.

“Her whole apprehension about school and change and whether or not we were going to be there at the bus stop to drop her off and pick her up — it all manifested itself in resisting the bus,” he said.

The bus driver began saving a seat for Grace in the front row. That helped her calm down.

This year, the first-grader marched up the steps on the first day but then froze. Two children were already in the front seat. It took a bit of intervention to clear Grace a spot. By the next day, a boy in the front row had a window seat reserved for her.

“As far as I’m concerned, the Miniature Hero of Bus 1112 can have my daughter’s hand in marriage,” Schrader wrote on the school e-mail group list.

Often, smaller riders are put in front seats, particularly on the way home, when some who are unused to long days tend to doze. Every few years, a bus driver finds a student asleep in row 11 at the end of the route. “I tell my bus drivers to be vigilant,” said Sutton, a retired military transportation officer who oversaw convoys in Kuwait and Iraq.

Christina wasn’t interested in the front. When she boarded, she marched all the way to the back, far away from her sister. Audrey shrugged. “She likes being brave and confident,” she said of her sister. Audrey was glad to save a place in the third row for her friend Lila, who was a few stops away.

From the back, Christina’s blond hair was barely visible over the tall seats. Her voice was mostly drowned out by the diesel engine.

“Look how high we are,” she said while staring out rain-drizzled windows at the front yards and houses going by.

The bus lumbered down narrow roads as it made its way toward a Spanish-English magnet program at Francis Scott Key Elementary School. It maneuvered around parked cars, landscaping trucks and at least a dozen other school buses.

“It’s like a roller coaster,” Christina said.

Over the 30-minute ride, more than 10 children boarded, but none joined Christina.

She sang quietly to herself and practiced addition: “One plus one is two, two plus two is four, four plus four is eight.” Mostly, she watched the world go by in a maze of wet streets, shopping centers and houses.

“Everybody ready?” the bus driver asked when she pulled up to a building where the sign read “Escuela Key.”

“This is Key School,” Christina announced. And she headed down the aisle toward her first day of kindergarten.