Several years ago, her district strictly enforced requirements that every student annually document residency. The district tried posting a photographer to snap students at the crossing but has since stopped that because it was difficult to identify the students, Castro said.
David Hinojosa, an attorney for the Mexican-American Legal Defense and Educational Fund, said he's concerned about students being singled out because they were on an international bridge before school.
Cooper, who conducted similar port-of-entry checks several years ago when he led the district in Nogales, Ariz., said no Del Rio students have been expelled so far.
Bob Dane, a spokesman for the Federation for American Immigration Reform, said Cooper's bridge stakeout prevented parents from taking advantage of a "duty-free education."
"It's very obvious the parents are cheating the system. The kids are getting quality education without contributing," he said.
Texas Education Agency officials know that most border communities have some students surreptitiously commuting from homes in Mexico, but there's been no recent effort to count them, said spokeswoman Debbie Ratcliffe.
"It does cost us to educate these children, but we also get a benefit because we know they are likely to impact our economy in some way," said Ratcliffe, noting that many will work in the U.S. as adults.
One of Texas' largest school districts, which is in El Paso, checks residency when students enroll, but spokeswoman Berenice Zubia said officials don't look for students at the international crossings that come from nearby Ciudad Juarez, Mexico.
Some parents in Del Rio say they're not taking any chances their children could be expelled.
Minerva Garcia, 50, hoped to move to her family's home in Ciudad Acuna to save money.
"If the students are willing to get up early to get across, it shouldn't be held against them," said Garcia, as she waited to pick up her 5-year-old and 8-year-old from school. "But I'm not going now."