Samuel W. Tucker Elementary School was built 11 years ago to fit 500 students. On Wednesday, Tucker, which operates on a modified calendar, will begin the new school year with 774 students.

"And I'm turning away another 100," says Morton Sherman, superintendent of Alexandria City Public Schools.

Following a decade of stable enrollment, the small Virginia school district has been battling such stark enrollment growth since 2008 that it is beginning to turn students away from their neighborhood schools.

The urban school system is running out of space to build portable classrooms and is resisting increasing class sizes, which are among the smallest for elementary-school students in Virginia.

Earlier this summer, school officials had predicted that 3.1 percent more students, or 12,800 children, would enroll in ACPS. As of Monday, 13,100 students have signed up for the new school year, and Sherman said he expects another 100 students to sign up for kindergarten before most schools open in September. As of last year, enrollment had grown 17 percent since 2008.

The problem is good in certain ways, Sherman says. Increased development cropping up around the planned Potomac Yard Metro station, and behind the Target, is bringing in more families. And what's more, "Families are no longer going to Alexandria schools K-5 and then moving out to Fairfax [County] for a lawn and bigger houses," Sherman says. The number of sixth-graders in ACPS middle schools increased from 810 to 964 in one school year. "The urbanization of families is occurring."

But it remains a problem. The school system expects to honor only about half of the hundreds of requests for transfers as it has in past years, meaning many more families wanting to send siblings to the same schools, or single parents trying to send their children to schools near their jobs, are being turned down.

Children whose neighborhood schools are at capacity will be allowed to return in future years if space opens up, but the school system is predicting increases of 5 and 6 percent for years to come.

In the next few years, three elementary schools will expand to serve grades six through eight. The school system is lobbying for at least two new schools, one on the west side of Alexandria, and one on the east.

Mt. Vernon Elementary School will open alongside Tucker on Wednesday -- both start earlier and end earlier than the other schools, with opportunities for extended classroom time -- with 778 students. Hal Cardwell, who has two children at Mt. Vernon, remembers when his third and oldest child began at the school in 2005, with about 300 fewer classmates.

At twice-annual music concerts, "We can't fit all the parents and the children in the auditorium -- there's just no way," Cardwell says. "It's a fire hazard."

His children love the school, but Cardwell says he's happy that the music concerts are split into two showings.