About 2,700 students pour into the hall after every class at Gar-Field Senior High School, forming a sea of laughter, loud conversation and backpacks.

Six minutes later, all of them -- or nearly all -- are sitting in classrooms. The scene is so normal that it's easy to overlook the year-long engineering feat that created it.

Gar-Field opened in Woodbridge in 1972 during the height of an architectural trend that emphasized open learning spaces over traditional classrooms, a design adopted in growing school districts across the country. The idea was to encourage students and teachers to mingle and collaborate during informal lessons driven by students' interests rather than rigid lesson plans.

Now, many of those school districts are putting millions of dollars into putting up walls and doors and closing up those open spaces permanently. They turned out to be too noisy for teaching, too hard to secure in an era conscious of crime and terrorism and unsuited to the current educational focus on standardized instruction and high test scores.

Now nearly complete, the work at Gar-Field is costing about $5 million, and Prince William County is pushing to enclose the other seven open-plan schools -- three high schools and four elementary schools -- by 2007. Lake Braddock High School in Burke, one of the largest open-space schools in Fairfax County, is about to undergo a $53 million renovation that will include closing the open spaces. Lewis Rauch, director of design and construction services for Fairfax schools, said that eventually all of Fairfax's six remaining open-plan schools will be converted.

Gar-Field teachers and students say they're thrilled with the results.

"It feels tremendous. I'm like a little kid with a new bike. Or a grown man with a new car. One of those," Principal Roger Dallek said. With new lights, lockers and classroom lab space, he said, it's almost like having a new school.

"It's much better than it used to be," said senior Chris Vann, 18. "It gives you that real classroom feeling," with no more distractions from the other side of thin partitions.

The open classroom trend was not an architectural anomaly. Larry Cuban, an education professor emeritus at Stanford University and former superintendent of Arlington County schools, said it was supposed to promote informal teaching: Students would work in small groups on projects of their choosing, he said, and teachers would be facilitators, working in teams, instead of lecturers.

"The architecture had a philosophy of education behind it," Cuban said. At the extreme, schools would have no lectures, no tests and no standard curriculum.

But teaching styles did not change to accommodate then-state-of-the-art classrooms. Chris Hamill, chairman of the Science Department at Gar-Field, taught at the school when it opened. Her classroom had a waist-high divider separating it from another room of students. Hamill recalled a colleague asking her to stop in mid-lecture because he was giving his students a test on the same subject.

"The kids would constantly be looking over at the other class to see what they were doing," Hamill said.

Within a few years, science classrooms were walled off to prevent potential hazards from spreading among classrooms, but the rest of the school still had to make do with makeshift barriers. Most used movable room dividers. In the meantime, the educational movement toward open classrooms had fizzled out by the late 1970s, replaced by a back-to-basics trend, Cuban said.

"Frankly, we had acclimated ourselves to it," said Dallek, the principal. But the new walls "make a huge difference. It has a totally different feeling."

During construction, classes rotated from their usual locations to 10 trailers in front of the school. When one set of classrooms was renovated, those students returned and another group moved outside. Only the science classrooms remain to be renovated, and they will require less work because they already were enclosed.

Enclosing classrooms has had the additional benefit of making it easier to move through the school, said senior Cliff Martin, 17, president of the Student Council Association. Before, students would spill out into aisles around the partitions and then work their way to the main corridors.

"There would be no movement at all. It would be like standing in an elevator," Martin said. The new halls direct traffic flow better, he said.

Now that the high school has new doors, it will be putting them to good use. The principal, teachers and several students went before the School Board recently to ask for a new tardiness policy.

Students used to be able to hang out in the vicinity of their classroom space and dive into their seats once the bell rang, teachers say. "Seventy percent of our [discipline] referrals are tardies," said electronics lab teacher Clarence Demory, who attended the board meeting.

But no more. Starting next year, when the bell rings, tardy students will find those brand-new doors closed. And locked. Instead of sliding into a seat, they'll have to troop down to the office.

"There's just something about a closed door," Demory said.

Classrooms with doorways are new for most Gar-Field teachers, such as Paul Wiesner, a special ed instructor.