CALEXICO, Calif. — Children are more apt to shield their faces than to smile when Daniel Santillan points his camera.

Santillan's photos aren't for any picture album or yearbook — they help prove that Mexicans are illegally attending public schools in this California border community.

With too many students and too few classrooms, Calexico school officials took the unusual step of hiring someone to photograph children and document the offenders.

Santillan snaps pictures at the city's downtown border crossing and shares the images with school principals, who use them as evidence to kick out those living in Mexico.

Since he started the job two years ago, the number of students in the Calexico school system has fallen by 5 percent, from 9,600 to 9,100, while the city's population has grown by about 3 percent.

"The community asked us to do this, and we responded," said board President Enrique Alvarado of the Calexico Unified School District. "Once it starts to affect you personally, when your daughter gets bumped to another school, then our residents start complaining."

Every day along the 1,952-mile border, children from Mexico cross into the United States and attend public schools. No one keeps statistics on how many children make the trek.

Citizenship isn't the issue for school officials; district residency is.

The U.S. Supreme Court has ruled that illegal immigrants have a right to an education, so schools don't ask about immigration status. But citizens and illegal immigrants alike can't falsely claim residency in a school district.

Enforcement of residency requirements varies widely along the border. Some schools do little to verify where children live beyond checking leases or utility bills, while others dispatch officials to homes when suspicions are raised.

Jesus Gandara, superintendent of the Sweetwater Union High School District, with 44,000 students along San Diego's border with Mexico, said tracking children at the border goes too far. "If you do that, you're playing immigration agent," he said.

The El Paso Independent School District in Texas sends employees to homes when suspicions are raised. But spokesman Luis Villalobos said photographing students at the border would be a monumental, unproductive effort.

That's not the thinking in Calexico, a city 120 miles east of San Diego that has seen its population double to 38,000 since 1990. A steel fence along the border separates Calexico from Mexicali, an industrial city of about 750,000 that sends shoppers and farm laborers to California.

Calexico's rapid growth outstripped school resources, resulting in overcrowding and prompting demands that Mexican interlopers be ousted. Taxpayers complained that their children were being bused across town because neighborhood schools were full, even after Calexico voters approved a $30 million construction measure in 2004. Portable classrooms proliferated.

The 62-year-old Santillan was hired in 2005. He is an unlikely enforcer. The Vietnam War veteran and labor activist is an outspoken advocate of amnesty for illegal immigrants and fills water jugs in the desert for Mexicans who trek across the border illegally.

Some students taunt him. Friends have called him a hypocrite. Santillan reminds them that he is only enforcing school residency rules, not immigration laws. Still, he says, "you've got to have hell of a tough skin."

Find extensive coverage of immigration issues at azstarnet.com/border.