T.C. Williams High School in Alexandria, Va. (Matt McClain/The Washington Post)

When students return to Alexandria’s only city high school in three weeks, enrollment is expected to swell to more than 4,000 students.

Classes at T.C. Williams High, which has more students than any other high school in Virginia, spill into six mobile trailers. In the next decade, enrollment is projected to exceed 5,360 students. The overcrowding spawns crammed classrooms and congested hallways.

“You feel like you’re salmon trying to make your way upstream,” chief operating officer Mignon Anthony said. “It’s very, very tight.”

The reason: More young families are opting to remain in Alexandria rather than decamping for suburbs in Prince William or Fairfax counties, driving up the city’s school-age population, said Hamilton Lombard, a demographer at the University of Virginia’s Weldon Cooper Center for Public Service.

Enrollment has declined in many Virginia school systems because of lower birthrates and migration. But, Lombard said, some districts — including Falls Church and Harrisonburg, near Charlottesville — have defied that trend.

Between 2011 and 2015, the number of Alexandria families with school-age children grew by more than 3,000, according to a report commissioned by the school district. The number of children 17 and under rose nearly 16 percent, to 25,557.

But before Alexandria City Public Schools considers opening a second high school or expanding T.C. Williams, the system will weigh another question: What new programs should it offer high school students?

Alexandria leaders say tackling that question before designing additional school buildings will allow the system to construct spaces best suited for the new programs.

The school system, which began a months-long review Monday of the crowding crisis, plans to develop programs for students based on local and national business needs. School system officials said they will determine those needs over the coming months by conducting focus groups with students, businesses, families and employees.

Alexandria will then decide the types of buildings and classrooms needed to accommodate the new programs. Alexandria officials said they hope to have a plan by May and new facilities in time for the school year starting in fall 2023.

“We’re turning the conversation into something that is more visionary . . . rather than just solving the question of numbers and students and seats,” school district spokeswoman Helen Lloyd said.

The district has looked for inspiration as far as High Tech High, which opened as a small charter school in San Diego and grew into a network of schools. Alexandria has also examined other systems that have just one high school, Anthony said. The school system may also look to forge public-private partnerships, she added.

The dramatic growth in Alexandria’s school population led to the construction of Ferdinand T. Day Elementary School, which focuses on science, technology, engineering and mathematics. It is to open this academic year.

Last school year, more than 15,800 students were enrolled in the school district, and more than two dozen classroom trailers were spread across the city.

Superintendent Gregory Hutchings Jr., who graduated from Williams and earlier in his career worked as an administrator in the Alexandria system, vowed to address high school overcrowding shortly after rejoining the Northern Virginia school system in July. At the time, Hutchings left open the possibility of building a new high school or expanding T.C. Williams.

T.C. Williams, which is divided between two campuses, opened in 1965.