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. 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 L...