A student walks between portable units at Barnsley Elementary School in Rockville. (Bonnie Jo Mount/The Washington Post)

Ten portable classrooms spread across the playground and grassy fields at Lucy V. Barnsley Elementary School, a sign of ever-increasing crowded schools. The Rockville campus has more students than it has capacity, so every fourth-grader spends most of the day in a trailer.

“They just keep adding them,” PTA mom Robbin Brinkhoff said.

At a time of rising enrollments and strained capital budgets in much of the Washington area, portable classrooms have new staying power. More than 2,000 of them dot the region’s public school landscape, set up near buildings and ballfields, at the edges of blacktops and parking lots — a seemingly permanent fixture of modern school life.

For many school districts awaiting new structures, portables ease crowding quickly at a relatively low cost. But they are often not as temporary as school officials intend, and some parents worry that children are more cramped and more isolated than in traditional brick-and-mortar classrooms. In a portable, a visit to a restroom can mean bundling up in a coat and being out of the classroom for 10 or 15 minutes.

“I think it seems to parents that portables should only be there a year or two, and they end up being there much longer,” said Cheryl Peirce, a longtime PTA leader in Montgomery County who advocates on capital improvement projects. “Their idea of temporary is a much shorter time frame.”

Where the portable classrooms are in the D.C. area

Thousands of students take classes in portables across the region. Fairfax County’s numbers are at an all-time high, with more than 1,100 trailers. In Arlington County, portables have proliferated: 121 are now in use, up from 29 in 2008.

The numbers in other districts have been steadier. Montgomery County has more than 450. Add to that about 500 in Prince George’s County and 200 in Prince William County. There are 56 portables on D.C. public school campuses, officials say.

“They’re not going away,” said David Lever, executive director of the public school construction program at the Maryland State Department of Education, which counted 2,900 portable classrooms throughout the state last year. “It would take an extraordinarily large capital program to make them go away.”

Still, portables endure with some begrudging acceptance. Maryland Gov. Martin O’Malley (D) has described them as “temporary learning shacks,” at odds with his vision of a 21st-century classroom.

Nationally, the most recent federal figures show that about one-third of public schools had at least one on-site portable classroom as of 2005.

Jeff Platenberg, Fairfax’s chief operating officer, said his district hit a high this year, a reflection of increasing enrollment and more programs that require space, such as those geared to early childhood education. The space crunch also comes as the district balances competing needs: to build new capacity and to renovate aging schools.

“Clearly, it is not desirable,” Platenberg said. “I think it presents a challenge for our teachers, but I think our teachers do an amazing job.”

Fairfax School Board member Megan McLaughlin (Braddock) says portables are a particular concern in elementary schools, because young students stay with one teacher in one room most of the day.

“That’s a long time to be in a very tight, confined space with limited natural light coming in through the windows,” she said. “I don’t think that’s conducive to them being engaged in the learning process.”

In Arlington, John Chadwick, assistant superintendent for facilities and operations, said “relocatables,” as his district calls them, are unavoidable, given a large influx of students, an immediate need for classroom space and a lack of vacant land. Since 2008, Arlington has bought units that include restrooms, he said.

“They’re an essential part of our solution, because we can get them in place for the next school year,” Chadwick said.

Although few people are thrilled about portables, he said, most prefer them to the alternative: larger class sizes. “There’s a balance between increasing class sizes and having relocatable classrooms,” he said.

Among parents’ greatest complaints is that many portables do not include restrooms, so children must walk to the school’s main building to use its facilities. This takes time and requires special security procedures, such as pairing up with a classmate or going in supervised groups.

The greatest challenges of portables may be transition periods and logistics, said Robert C. Pianta, dean of the Curry School of Education at the University of Virginia. “Facilities matter,” he said, “but they don’t trump a solid program and solid people delivering that program.”

Not all districts have embraced portables equally.

Alexandria has no portables, schools spokeswoman Kelly Alexander said. To meet its enrollment growth, the city added 20 “modular classrooms,” permanent structures that arrive mostly assembled and provide classroom space relatively quickly.

In Loudoun County, Superintendent Edgar B. Hatrick said he opted for new construction, limiting portables as the district grew rapidly. The district built 50 schools in 22 years and lived with overcrowding at times. It has 31 portable classrooms in all.

The number of portable classrooms in Montgomery has fallen since a peak in 2005, when there were several hundred more. Enrollment plateaued for a time and new school projects were built, so fewer portables are needed, said Bruce Crispell, director of the division of long-range planning.

Since then, enrollment has ticked up rapidly. Just this school year, Montgomery’s student population grew by more than 2,500 — enough to fill more than 100 classrooms.

“If our enrollment continues at this pace, I see it as hard to get away from,” said Michael A. Durso, who represents District 5 on Montgomery’s Board of Education. “It’s our only fallback.”

Portables may be a quicker fix than new construction, but they are not cheap. One-time set-up costs are $65,000 per classroom, said James Song, Montgomery’s director of facilities management. There’s an additional lease fee of about $5,000 a year, per classroom.

With enrollment projected to continue to surge in coming years, “for the near future, I think the number of portables is going to increase,” Song said.

At Barnsley Elementary, in Rockville, the four portables the school had four years ago became 10 as enrollment grew and the school was designated for a program to reduce class sizes for early grades.

With space for 411 students, Barnsley has an enrollment of 680. It ranks among the schools in Montgomery that have more portables.

Principal Andrew Winter said he would like to see the portables gone. A 12-classroom addition is included in a $1.74 billion capital improvement plan that the school board just approved and county officials will soon consider.

Even if the addition is approved, it will not be open until August 2017. In the meantime, portables have taken over half of the blacktop playground and part of the schoolyard. Many teachers say one big drawback is a lack of storage cabinets and cubbyholes that are common in typical classrooms.

Although some teachers like the quiet environment of portables, few prefer working in them, said Tom Israel, executive director of the Montgomery County Education Association.

Fourth-grade teacher Katie Shapot notes that portables in Montgomery don’t have the sinks that other classrooms do, which can mean a time-consuming trip to a restroom for science experiments. “It makes our science unit on ecosystems very difficult,” she said.

One of her students, Andrew Byrne, 9, also mentions that the long walk to a restroom as one of the disadvantages.

“The thing I don’t like,” adds Lilly Natchipolsky, 10, “is that it takes up some of the space we have for recess.”

But students talk about pluses, too. Portables have temperature controls.

And Brady Goyne, 10, likes the proximity to recess space. “I just think it’s a cool thing to have a basically outdoor classroom,” he said. “You can just go out of your room, and you’re already on the field.”

Renee Estrada, the school’s PTA president, said parents are hoping that the classroom addition project is approved — and that no more portables are added before is built. “We’re all collectively crossing our fingers.”