Inside a cavernous World War II-vintage hangar in Hagerstown, Md., where twin- engine AT21 attack planes were built 45 years ago, a Montgomery County elementary school is under construction.

The school is Strawberry Knoll, and by next September, children in a neighborhood north of Gaithersburg will fill its classrooms.

Strawberry Knoll is being built of "modules" -- large, manufactured components that are assembled at one place, then disassembled, trucked to a permanent site and reassembled there.

Modular construction has been used to build more than 50 classrooms in Montgomery schools in the past four years, usually in additions to existing schools or as adjuncts to new construction. But Strawberry Knoll, being built by Commercial Modular Systems, is expected to be the first all-modular school in the Washington area.

As such, it is an experiment, county school officials say -- a trial run to determine whether modular construction techniques can provide enough flexibility to overcome the tyranny of demographic changes, and whether its advantages make it worth the extra cost.

Fluctuating school enrollments have dogged county school planners for decades, forcing unpopular school closings in older neighborhoods with shrinking numbers of children and adolescents, and expensive school construction in booming upper county areas. But modular construction creates schools that can shrink or grow.

"We're in a period of rapid enrollment growth again," said Philip Rohr, director of Montgomery's educational facilities planning and development. "In the next six years, we'll be building 16 new elementary schools. The reason we're trying modular construction is that enrollments tend to go up and down."

When enrollments dip, modular classrooms can be moved to other locations where they are needed. But, unlike the trailer classrooms used in many area school systems to accommodate growth, modules are generally more accepted by parents. They can be arranged in rows along corridors, just like normal classrooms, and from the outside they blend in well with the original school buildings.

All new Montgomery schools, including Strawberry Knoll, have similar exterior facings of brick and glazed tile, detail that the manufacturer adheres to on the modular units.

"They want to be able to mix and match {modules} sometime down the road," said William French, vice president of the firm.

"It's hard to say when we might move some," said Richard Hawes, who heads the county school system's construction division. "But we have standardized our design so we can do it."

Strawberry Knoll, whose 30-plus rooms will be all modular, is the ultimate so far in modular school construction, according to French. Commercial Modular Systems has done millions of dollars worth of modular construction for the county since the method was first used in an addition to Gaithersburg High School in 1984.

Strawberry Knoll is now a collection of 118 structural steel-framed oblong sections being assembled by about 70 workers in a giant building where the former Fairchild Aviation Corp. once built military aircraft.

By June -- when the school's interior will be painted, the exterior faced with brick and tile, the wiring in place, the blackboards screwed on the walls, the cubbyholes built in kindergarten rooms, and the bookcases installed in the library -- the whole 52,057-square-foot structure will be broken down into shoebox-shaped components and trucked 60 miles south on a caravan of flatbed tractor-trailers.

At the site near Gaithersburg, workers will need about two weeks to reassemble the modules -- two to a classroom -- and attach them to a gymnasium and cafeteria being built conventionally by Dustin Construction Co.

Some years down the road, when the neighborhood around Strawberry Knoll no longer has the 740 young children it is designed for, the school can be disassembled and the classrooms moved to wherever they are needed. Only the gym and the cafeteria will remain to serve as a community center, according to Rohr.

The question now, Rohr said, is whether the advantages of modular construction -- flexibility, movability and speed of construction -- are worth its extra cost. "We are relooking at the cost-benefits," said Rohr. "We are paying a premium for this."

The cost of Strawberry Knoll's classroom space is running at $87 per square foot, according to Hawes. That is about 13 percent higher than conventional construction. The total cost of the school will be about $6.8 million, roughly half for the modular rooms and half for the core area.

"It was not apparent how much more expensive modular construction is until we bid Strawberry Knoll," said Hawes. And the true cost or saving of the method, he said, will be realized only when the modules are moved.

Those who teach in modular classrooms, which now make up 25 percent of all newly constructed county schools, generally like them. "It's nicer than most regular classrooms I've taught in," said Shirley Sheehan, a fifth grade teacher at Christa McAuliffe Elementary School in Germantown, which has a wing of eight modular units. "The only problem is that you can't tape things up on the walls, and that's a major problem." Tape damages the interior dry wall finish of the classrooms, she explained. Regular classrooms have masonry walls.

From the outside, Sheehan's classroom wing looks like the rest of the school, except for individual heating/air-conditioning units in the outside wall. Only a vertical seam on two walls indicates where the modules join.

Commercial Modular Systems is also building a classroom addition for Howard County, and French reported that a parade of school officials, some from as far as North Carolina, has visited the firm's plant recently.

In Fairfax County, the nation's 10th largest school district, which has also been plagued by shifting populations, there is a wait-and-see attitude toward modular units.

Fairfax relocated a seven-classroom modular building several years ago, and the cost was more than $100,000, according to Alton Hlavin, assistant superintendent for facilities. "We will be watching with interest, but at the moment we are building conventionally with long-term needs in mind," he said.