“Most of the time I am really, really tired,” said Martha, whose parents moved back to Tijuana because the cost of living was cheaper here than in southern California.

“I try to do my best,” she added. “But sometimes, I just can’t.”

In the raging debate over immigration, almost all sides have come to agree on tougher enforcement at the border. But nearly unnoticed, frustration is focusing locally on border-crossers who are not illegal immigrants but young American citizens, whose families have returned to Mexico yet want their children to attend American schools.

Called “transfronterizos,” these students migrate between two cultures, two languages and two nations every day, straining the resources of public school districts and sparking debate among educators and sociologists over whether it is in American interests that they be taught in the United States. Although some Mexican families pay the steep tuition required of out-of-district students, most do not, and many that pay taxes out of their paychecks do not pay the property taxes that support public services.

Some of the students’ parents are American citizens and some are Mexican.

Students like Martha fly under the radar in some school districts, while other districts assign truancy officers to find who they are. They live with the anxiety of potentially having to lie about their residency and the very real possibility that the prize they are after — a decent education — will be taken from them. Though their exact numbers are unknown, their presence reflects the daily complexities of border life — among them, economic and educational disparities between the United States and Mexico and families splintered by deportation and unemployment.