The children who spent the past school year in Jill Silton's third-grade class can't rely on their yearbook -- "Memory Lane 2005" -- if they want to remember all their classmates.

When Silton welcomed her class to McNair Elementary School in Herndon on Sept. 7, the first day of school, there were 20 students in the room, at least five of whom were new to the school. In June, she sent 20 children off for summer vacation. But it wasn't the same 20. In between, seven new students entered her class and seven children moved away. (Two of those children came and went.)

New faces are the norm at McNair, which has the greatest student turnover of any Fairfax County public school. Although final figures haven't been tallied for the past school year, more than 350 new students enrolled on or after the first day while many others withdrew, putting the student body around 940, school administrators said. One week in March, 10 students arrived and 13 left.

"I'll get a note in my box that says the next day you'll have a new student," Silton said. "I have to face these unknowns. How is their personality going to mesh with the rest of the class? What gaps are there going to be in their learning? There are a lot of blanks to fill in."

Teaching in schools where the student body fluctuates is an increasing challenge for educators nationwide trying to meet the demands of the federal No Child Left Behind act, which requires student performance on standardized math and reading tests to improve each year. Although the law doesn't require states to consider the scores of newly arrived students, those scores do count in following years. And each year's lessons provide a foundation for the next. Plus, studies have shown that teachers in schools with high turnover are more likely to fall behind on the year's lessons.

"Mobility tends to place some stress on schools," said Robert C. Pianta, an education professor at the University of Virginia. "The shift of kids in and out of the classroom interrupts the educational process. It impacts how well teachers get to know these kids . . . and creates a situation where teachers are sort of starting from scratch."

This year, McNair is the only high-poverty Fairfax school that didn't meet No Child standards for three consecutive years and must now allow children to transfer to other schools and provide private tutoring. High turnover is only one challenge at McNair, where 40 percent of children are learning English and 40 percent are from low-income families.

But Fairfax education officials note that McNair failed to meet the benchmark passing rates only because Hispanic students did not meet the mark in English. Overall student performance improved in English and math.

Teachers attribute some of McNair's success to classes and programs intended to ease the transition for new and old students. For example, students break into small groups for math and reading lessons so that each child can work with others at the same ability level. Children entering midyear join the groups that suit them best, and teachers don't have to repeat lessons for everyone.

McNair teachers find ways to fit in informal assessments to determine whether a child who arrives midyear can read or subtract as well as his peers. Sometimes, newcomers need to catch up in history and science classes, and other times teachers search for creative ways to include a student who has just finished studying ancient Rome in a lesson on the same subject that is just beginning.

But before a new student opens a book, Silton and her colleagues said, it is important to help him feel at home. Buddies are assigned to help newcomers navigate the hallways, and the school hosts potluck suppers so parents can meet. Counselors gather the newest students together in small groups to talk about moving to a new place and making friends.

Jane Awtrey, a fifth-grade teacher, said that if the newcomer doesn't speak English, she and her class learn to say "hello" in his native language. The smallest details, such as making sure a new student has a desk with a name tag, can make a difference, teachers said.

Gautham Ravichandran, who said he has lived in "about 17 places because of my dad's work with software," remembers joining Silton's class on Feb. 14, expecting that everyone would get valentines except him. "I was new, and they didn't know me," he said.

But he got a stack. Silton, anticipating the problem early that morning, had all the children make cards for their new classmate before he walked in.

One thing the teachers say they don't worry much about is making sure new students are accepted by their peers: Most McNair students know what it feels like to be the new kid.

On a hot June afternoon, most of the children in Silton's class started to play kickball on the school's basketball court during recess. Anita Pathammavong, who had transferred from another Fairfax school in May, lagged behind.

Moments later, Mohmed Mudather wandered over with a Frisbee. "Hey, Anita -- want to play?" he asked, drawing her into a game.

Anita, who emigrated from Laos in 2001, has been a student at four Fairfax schools because her family has moved several times in search of affordable housing. She said that her first few days at McNair were difficult but that it didn't take long to feel part of the class.

"It was kind of sad, because my best, best friend was at Bonnie Brae [Elementary School], and I kind of miss her," Anita said. "But it's kind of fun because I have lots of new friends."

In Fairfax County, where about 166,000 children attend school, about 12,700 enrolled sometime after the first week of the 2003-04 school year, the most recent year for which data are available. About 5,700 were new immigrants, private-school children or others who hadn't attended a U.S. public school that year. About 1,800 moved from public schools in another state. Nearly 4,000 switched schools within the county. During the school year, about 13,400 left Fairfax schools.

Other districts experience similar churn. According to a July 2004 report by the Maryland State Department of Education, more than a half-million students in the state -- about 13 percent -- transferred to a different school between 1998 and 2002.

Immigration fuels much of the flux in Washington area schools, officials said, because many families begin their lives in the United States by staying with relatives or friends before settling into an apartment or a home. With the high cost of housing, some families jump from rental to rental. Other children move because their parents change jobs or divorce.

Another of Silton's students, Tamanna Sahni, started at McNair toward the end of September after arriving from India, where she had been staying with her grandparents while her mother completed a master's degree.

Tamanna said she quickly made a friend who lent her books to read.

But this year, she'll attend school in Loudoun County, because her family moved from a Herndon condominium to a townhouse in South Riding.

Fizza Batool, 10, finds a cozy spot to read at McNair Elementary School in Herndon -- under a desk. In the foreground is Suhana Elamsenthil, 9.