When Mill Run Elementary School was under construction in 2000, principal-to-be Paul L. Vickers specifically requested that a chain-link fence be built to mark the line between his school and Eagle Ridge Middle School next door.

So it was symbolic when he asked this year that a piece of that fence be taken down so a new sidewalk could stretch unimpeded from one school to the other. For the next year, students and teachers will be moving between the two schools, as Mill Run's fourth- and fifth-graders attend classes in an empty wing of Eagle Ridge. The elementary school's building will house kindergartners through third-graders.

"It was sort of a 'Mr. Gorbachev, tear down this wall' moment," Vickers said.

The unusual situation is the result of unexpectedly fast growth in Mill Run's Ashburn neighborhood. To accommodate hundreds of new pupils, the Loudoun County School Board voted to open Legacy Elementary in the Brambleton subdivision in 2005 -- a year earlier than planned.

But relief will not come soon enough for Mill Run. School planners project that by the end of the year, Mill Run will have more than 1,300 students, far more than its capacity of 800. At that size, the school will be larger than all of the county's 11 middle schools and three of its eight high schools.

Vickers said that when he learned how many students were expected at his school, he immediately thought of the more than 400-projected empty seats at Eagle Ridge. The school temporarily shrank in size when Belmont Ridge Middle School opened last year.

Opening a satellite campus means that no Mill Run students will be declared overflow and have to attend other schools. (Last year, 63 students who registered late were sent to three schools where there was room.) Holding classes at the neighboring school also means avoiding unsightly and expensive trailers.

"This is the very, very best solution to what could be a very difficult situation," Vickers said.

Planning for the split school started in November and involved several parent meetings and an active transition team. To make Mill Run students feel at home in the larger school, Vickers has brought touches of elementary school to them. A Mill Run pennant hangs above every satellite campus classroom. The elementary students will have their own books arranged on their own labeled shelves in the Eagle Ridge library.

Administrators will work hard to limit interaction between the two age groups. Mill Run students will use the cafeteria at different times than the middle-schoolers and play in Eagle Ridge's auxiliary gym. A middle school guidance office has been refitted as a clinic, where fourth- and fifth-graders can have health issues sorted out away from older children. Former third-grade teacher Gregory West has been assigned as a full-time assistant principal at the school, working out of an office at Eagle Ridge.

Mill Run's buses will stop at both schools, depositing fourth- and fifth-graders 10 minutes early at the satellite campus. Only students who usually walk to school will be allowed to traverse the new sidewalk between the buildings, and then only accompanied by adults.

Vickers said the level of detail that had to be worked out was consuming -- everything from ordering elementary school desks and chairs for the Eagle Ridge building to installing a telephone in a storage closet to transform it into a teachers conference room.

"The satellite campus is not an afterthought," he said. "There's been nine months of forethought put into it."

Several parents said they appreciated Vickers's attention to their concerns, but they said they wished Legacy had been built more quickly to avoid splitting Mill Run's students.

"They've tried to put our minds at ease," said Nancy Hill, whose daughter will be in fourth grade at Mill Run. "We'll have to wait and see."

Sue Morris, whose husband spoke out against the idea at a School Board meeting last year, said she has come to see the satellite campus as the best option.

"It's a better solution than splitting up the kids all over Ashburn," she said.

But Morris said the attention to detail has not erased all worries. For instance, she said, she remained opposed to a plan to give fourth- and fifth-graders lockers usually used by middle school students.

"A lot of people aren't ready for that," she said. "People hide things in lockers. Fights start at lockers."

West said school officials would wait to see how things go at the lockers. He noted that the elementary students would not be given time to congregate at lockers during the day but would be expected to visit only in the morning and at the end of the day. And teachers will be watching carefully for any combination problems.

"We've got a key that opens all of them," he confided.