Arlington County and City of Alexandria public schools are overflowing with new students. To deal with the growing enrollment, both school systems have added new classrooms and are building new schools.

An expected 12,381 students will attend Alexandria’s schools in the 2011-12 school year, a 3.2 percent increase over last year’s enrollment, said Deputy Superintendent Margaret Byess. The majority of that growth is in the elementary schools, accounting for more than half of that projected enrollment, she said.

Arlington County public schools officials project that 22,245 students will fill the halls this school year — 5.5 percent more than last year, said Linda Erdos, a schools spokesman.

Arlington installed a total of 28 temporary classrooms, 17 of which were relocatable classrooms put in at five schools. Internal reorganizations created another eight classrooms, said John Chadwick, Arlington’s director of design and construction.

Portions of the new $65 million Yorktown High School are open, and the second phase should open by December. The third phase will open by fall 2013, he said.

This fall, students there will have a new cafeteria, a refurbished auditorium and more classrooms. A new aquatic center, gymnasium and black box theater will be among the improvements coming in the winter, Chadwick said. By 2013, a new media center and more classrooms will be built and open about the same time Wakefield High School’s first phase of renovations starts.

Construction of the phased $115 million Wakefield High School has begun, he said. The first phase, which will include an aquatic center, new gymnasiums and classrooms, is expected to be complete by fall 2013. The second phase, which includes demolition of the existing school building, new athletic fields and installation of a geothermal well field, should be complete by fall 2014, Chadwick said.

“We are looking at having as many students in the system within a few years as we did in the late ’50s, early ’60s.. . . That is quite something — and we don’t have as many school buildings now as we did then,” Chadwick said.

School construction must “be smarter” now, he said.

“We need to be more reasonable and make sure all of our buildings are being used as fully as they can without jeopardizing our goals,” Chadwick said.

Computer labs have been made into classrooms, admissions policies for specialty schools are accepting more children, and some class sizes have increased, said Abby Raphael, Arlington School Board chairwoman.

“We have really been looking at many different ways to use the space we have well and efficiently, but we clearly need more seats,” said Raphael, who said school staff members are gathering data on where additions can be built, where other efficiencies can be made and on cost estimates for future capital projects.

Alexandria is using a modified open enrollment program to make the most efficient use of its school buildings. Elementary classes have caps of 20 to 24 students per class. Once a school reaches capacity, students are bused to a nearby school with space.

To add to that capacity, modular classrooms were built off-site and installed on foundations as additions to existing schools. Last summer, John Adams Elementary School received several of these classrooms. This summer, 20 more were added: 10 at Patrick Henry, five at James K. Polk and five at Charles Barrett elementary schools, Byess said.

“Between the modular classrooms and modified open enrollment, we are using our space very efficiently,” Byess said. “It is less expensive to transport the children than it is to hire an additional teacher and build an additional classroom.”

Modular classrooms won’t be able to handle the future needs of the school system, and new construction is in the works.

Jefferson-Houston School, which houses students in pre-kindergarten through eighth grade, has city funding and will have education specifications, project management and architectural and engineering service contracts in place by October. A new school is set to open in fall 2014, she said.

Patrick Henry Elementary also received city funding, and the planning and construction process will begin next year, she said. The new school, which will serve students in pre-kindergarten though eighth grade, will be built next to the existing one. It is scheduled to open in fall 2015. The old school will be demolished, depending on enrollment at the time.

The Alexandria City Council and School Board are working on securing funding for two more schools, a reconstruction of Cora Kelly Elementary into a pre-kindergarten to eighth grade, and a new school with a site to be determined, she said.

“If we keep continuing at the rate we have over the past few years, we won’t be able to close [the old Patrick Henry] until the fourth one is opened,” Byess said.