Student enrollment in Fairfax County schools surged past 180,000 for the first time as classes resumed Tuesday, a development that poses major logistical and academic challenges for the region’s largest school system.

Over five years, the student population in the county has climbed more than 9 percent to an estimated 181,500. That translates to an additional 15,000 students since 2007, or more than the entire enrollment of Alexandria public schools.

All of that growth means a 1,500-bus fleet is shuttling ever more students to class on congested Northern Virginia roads. Portable classrooms are mushrooming on campuses. Teachers and principals are scrambling to ensure that growing numbers of students from poor families or from homes where English is a second language get the help they need.

Superintendent Jack D. Dale, scheduled to retire in June after what will be a nine-year run, said the student surge “has exacerbated our problems, and the result is a strain on some of our schools.”

Enrollment was also projected to rise in Prince William County, Arlington County and Alexandria, where classes also resumed Tuesday. In Loudoun and Montgomery counties, where schools reopened last week, enrollment is climbing. (Loudoun has had the fastest five-year growth rate in the region — about 25 percent.)

In District schools in recent years, most growth has come in public charter school sector.

Prince George’s County schools have been an exception to the regional trend: Enrollment has slid in recent years.

In Fairfax, the complications of growth are on display at Bailey’s Elementary School for the Arts and Sciences, a two-story, red brick building near Baileys Crossroads in the Falls Church area.

County School Board member Sandy Evans, whose Mason district includes Bailey’s Elementary, called the school “grossly overcrowded.”

About 1,300 students were expected in class Tuesday, from pre-kindergarten through fifth grade. There are 19 portable classrooms — more than at any other county school — zigzagging behind the main building over asphalt where students used to play foursquare and hopscotch.

The modular buildings compose a veritable scholastic trailer park that was erected on campus in recent years to accommodate all of the students in what data show is the region’s largest elementary school.

Elsewhere, Fairfax opened two new schools: Mason Crest Elementary, for 700 students; and South County Middle School, on the site of the old Lorton prison. South County has a capacity of 1,300.

The early morning scene outside Bailey’s, with hundreds of parents and children lined up almost 100 yards down the block, more closely resembled opening day at Nationals Park than the start of classes at a suburban school. Families crammed shoulder-to-shoulder through doorways, nudging past teachers who directed traffic toward classrooms.

“I know a lot of students will be overwhelmed as they walk in,” said Keith Hall, the school’s interim principal.

Despite the school’s size, Hall said the goal is to “maintain doing the kinds of things that traditional elementaries have, but we just have to be creative in the way we do them.”

To make room in the cafeteria, a series of lunch periods begin before 11 a.m. Kindergartners and first-graders are expected to eat in their classrooms to help ease lunchtime congestion.

Families and students seemed unfazed. Suzie Phipps, a parent, said the school benefits from a diverse community. Most of its students qualify for free or reduced-price lunches.

“I don’t see the space crunch as something that would drive me away from the programs at Bailey’s,” said Phipps, 40, who has twin sons in fourth grade. “The opportunities to learn from every walk of life outnumber the capacity crunch.”

Students at Bailey’s speak more than 20 languages, and signs in the school have instructions in English, Spanish and Arabic. It is also common to hear kids chatter in Urdu, Dari and Hindi, officials said. More than half of Bailey’s students are Hispanic, one of the county’s fastest-growing groups.

Larry Bizette, a school system demographer, said enrollment has been fed in part by high birthrates among minority county residents and a downturn in the housing market.

Bizette said that the Hispanic birthrate is almost triple that of white non-Hispanics.

In addition, Bizette said, fewer parents have been able to buy homes in the outer suburbs, farther away from the District. The slow economy, he said, made it difficult to sell condominiums, townhouses and single-family homes, so many families remained in the area and enrolled in local schools.

The enrollment at Bailey’s, up at least 100 students this school year, is also driven by school policy. Bailey’s became a magnet school during the early 1990s to attract more children who were proficient in English.

Kevin Sneed, director of design and construction services for county schools, said the Bailey’s building is 17 percent over capacity. His team added two portables this year for additional space for a total of 19 on the campus. Sneed said there is no more room for more.

“We are struggling to find space — absolutely,” Sneed said.

Officials said a number of options are being considered to make room for Bailey’s rising enrollment, which is projected to exceed 1,600 in five years.

One radical option, Sneed said, would be to remove the school’s library.

“I don’t ever want to do that,” Sneed said. “It would be hard to live with myself.”

Christine Adams, a PTA activist and mother of a fifth-grader, said some people have suggested adding a third story to the school.

“There’s no obvious solution” to the space crunch, Adams said.