WASHINGTON – Schools are under orders to make sure no child is left behind, but that can be tough to do when so many children are moving.

At least four in 10 students change schools at least once by the time they are 17, on top of their normal progression from elementary and middle schools to high school.

Students typically switch locations for reasons involving their parents – from job changes and marital breakups to military assignments and seasonal work for migrants.

The moves mean millions of children must adjust not just socially but academically, particularly when they switch midyear and cross state lines. Each state chooses its own curriculum, testing and definition of success.

When Jenna Gosser’s family moved from San Diego to Saginaw, Texas, on Oct. 1, she was studying the colonies. Her new class was past that already, and had moved on to the Bill of Rights.

Jenna, 13, also wound up in the middle of a lesson about a writer she barely knew, Edgar Allan Poe. And she had no preparation for proportions, the topic in her math class.

“I’m trying to catch up, and it’s getting easier,” said Jenna, an eighth-grader at Highland Middle School in Saginaw, a Fort Worth suburb.

The Gossers made the choice many families do: They moved for a better life, even if it meant short-term struggles at school for Jenna and her younger sister, Taylor.

Both girls say their teachers have given them extra support, delighting their parents.

At Fort Belvoir Elementary, a public school on an Army post in Fairfax County, Va., hundreds of its 1,300 children come and go during the year. As military dependents, they arrive from far-flung places – including Panama, Germany and Alaska – that have varying academic standards.

So the Virginia school has after-school programs, Saturday classes and volunteer help from the military to aid students.

Children in military families often have more support than others who move often, said Russell Rumberger, a professor of education at the University of California, Santa Barbara.

His research in California found students who changed high schools even once were less than half as likely to graduate than other students, even when controlling for other factors. They also were more likely to have trouble making friends and less likely to participate in after-school activities, Rumberger found.

Even students who didn’t move were influenced, as the turnover around them affected classroom instruction and teacher morale.Schools, under pressure to make yearly progress under the No Child Left Behind law, have a cushion when it comes to student mobility. They only count the test scores of students who have been with them for a full academic year.

The same is true for school districts and states. The idea is that schools should not be judged on the progress of children they have had little time to teach.

But such flexibility also means no one is really accountable for many students who move during the year, said Katrina Kelley, director of the Council of Urban Boards of Education for the National School Boards Association.