By 9:15 a.m., a crowd of students was gathered outside the doors to the George G. Tyler Elementary School, a giggling, squirming, waist-high sea of humanity ready to surge through the entrance.

Student helpers held numbers on posters high above their heads to show where children in each grade should line up. The halls would be swamped if all the children entered at once, so they usually are allowed into school grade-by-grade to avoid a bottleneck.

A few minutes before class started at 9:30 a.m., the teachers let the students stream in. Within minutes, the corridors were nearly empty, the students already inside the classrooms or the seven trailers parked behind the school. Tyler, joked one longtime staff member, has "elastic walls."

That's a typical day at Prince William's most crowded school.

The Gainesville school doesn't have the most students: Cedar Point Elementary, south of Tyler off Linton Hall Road, holds that honor this year, with more than 1,100 students. It's also not the smallest; elementary schools such as Yorkshire in Manassas and Bel Air in Woodbridge were built when schools designed for 400 students were sufficient. Newer elementary schools accommodate twice as many students.

What Tyler has is many students in a small space. About 730 students attend class in a school built for 472, putting it at 155 percent of capacity. That's less than the 835 students Tyler had two years ago. The school opened in 1968.

"The classrooms are . . . always willing to accept one more," said Michele Salzano, who is in her first full year as principal. She was assistant principal for 11/2 years and on administrative assignment for two years at Tyler.

Tyler has grown, shrunk and grown again as new schools were built in the western end of the county. Mountain View, Bristow Run, Cedar Point and Alvey elementary schools have all pared off part of Tyler's boundaries, but the new houses, and new students, keep coming.

When new elementary schools open in the Glenkirk and Victory Lakes neighborhoods next year, Tyler will likely shrink again, Salzano said. But she expects enrollment to creep back up by 2006.

"This end [of the county] is just booming," she said.

Preparation is the school's first step in coping with those changes. The county first estimated Tyler should expect about 570 students this year. Salzano thought that was way off and planned to hold on to teachers for the students she expected.

Salzano anticipates adding a fourth-grade teacher, and when that teacher starts, there will be five classes at each grade level.

The crowding requires well-choreographed maneuvers to avoid waits. Students move among classrooms in small groups so that halls don't get clogged. Educational materials are hung on the walls, so students always have something to read as they walk by.

Lunch shifts start early and end late to accommodate everyone. Students bring books to read as they wait during classroom bathroom breaks. Volunteer-run reading groups sneak in extra work in a quiet stairwell. Children are encouraged to visit the library throughout the week, rather than at a set time, to avoid packing into the room.

"It's controlled chaos, and they do well," librarian Nancy Thaete said.

Deborah Foulk-Libby, a teacher at Tyler since 1997, said the large population can have unexpected benefits. One year, when the fourth grade was exploding, her second-graders each had two fourth-grade reading buddies instead of one.

"They just had another person to look up to," Foulk-Libby said.

Some students serve as escorts for new ones, so they'll have at least one familiar face from their first day.

"At my old school we didn't have anything like this. It's pretty cool," said 10-year-old Richie Shannon, a fifth-grader and greeter. He takes the extra students in stride. "A lot more people, a lot more friends."

Parents said they're used to change and growth as a constant at the school.

"I'm sure every parent would love to have 15 kids in a classroom, or 20, but we don't have that capability. And it works out great," said Sandi Testut, the PTO co-president and mother of a Tyler fifth-grader.

Chris Trone, who has a second-grader and a fifth-grader at the school, isn't sure the boundary changes will affect her.

"I'd love to stay, but I've heard to learn to be really flexible," said Trone, who works in the school's office. "We just have to work together."

Students at Tyler Elementary line up at the entrance, waiting to be admitted. The school opened in 1968; it was built for 472 students and now has about 730. Volunteer Melaina Kupka helps second-graders, from left, Alexus Canfield, Jessica Blanchard, William Olsen and Amani Colvin with their reading under a stairway, one of the few spaces available to them in the crowded school.Robin Helm's third-grade class reads in the hallway while waiting for a bathroom break. The crowding requires well-choreographed maneuvers to maximize time and space. Above, seven trailers behind the school help ease crowding. The school wants to add three trailers to house art class and more students. Right, with 29 students, Rosemary Womack's fourth-grade class is large even by Tyler standards.