In Los Angeles public schools, transgender students have equal access to facilities like bathrooms. They can play on sports teams that align with their gender identity. Teachers and counselors consult with students before discussing their gender identity with parents, in an effort to prevent unwanted disclosures.

All of those practices are required by school district policy. But those rules cross a red line for Donald J. Trump, who has said that if he returns to the White House, he will withhold federal funding from schools that recognize transgender identities or teach what he calls “gender indoctrination.”

If that threat was realized in Los Angeles, for instance, the effects would be immediate, according to Alberto Carvalho, the superintendent of the 500,000-student district. While only about 8 percent of the district’s funding comes from the federal government — in line with other systems nationwide — the amount of money, $861 million this school year, is huge.

Withholding all of that would require bipartisan action from Congress and is highly unlikely. But by rewriting federal civil rights regulations about sex and gender, a second Trump administration could apply significant pressure on schools in left-leaning regions like Los Angeles and New York. Districts could be investigated by the federal government and required to change their gender policies, under threat of legal action or fines. Smaller amounts of federal money could also be withheld through presidential executive action.

Still, Mr. Carvalho said he had no plans to rethink the district’s policy on gender identity, even if Mr. Trump is elected.

“These are core values that will remain,” he said.

Currently, federal funding in Los Angeles supports some of the city’s most vulnerable children, including those in poverty and those with disabilities.

The money contributes to the salaries of teachers, which can help lower class sizes in schools where students are struggling academically, Mr. Carvalho said. It allows the district to hire more tutors, school counselors, social workers and psychiatrists. It also sometimes pays for devices and home internet connectivity for students who would not otherwise be able to log into online assignment portals.

For some disabled students, federal dollars support personal aides who attend class alongside them. It helps the district purchase adaptive technology, like voice output devices, which can aid students with speech and language disabilities.

And for the most severely disabled children, the money can be used to provide teaching or tutoring in their homes.

Losing federal funds would not necessarily mean cutting these programs entirely; states are required to supplement the money that flows through Washington. But in a district like Los Angeles, where 92 percent of schools enroll large numbers of low-income children and qualify for significant federal support, Mr. Carvalho said the loss of the dollars would be “devastating.”