LOS ANGELES — More than 30,000 Los Angeles public-school teachers began the largest school strike in the country Monday and the first in three decades in the district. Holding plastic-covered signs on rain-drenched picket lines across the city, they demanded higher pay, smaller classes and more support staff in schools.

The strike effectively shut down learning for roughly 500,000 students at 900 schools in the district, the second-largest public school system in the nation. The schools remained open, staffed by substitutes hired by the city, but many parents chose to keep their children at home, either out of support for the strike or because they did not want them inside schools with a skeletal staff.

With negotiations apparently at a standstill, the strike could last days or even weeks.

The decision to walk off the job came after months of negotiations between the teachers’ union, United Teachers Los Angeles, and the Los Angeles Unified School District. Although educators on all sides agree California should spend more money on education, the union and the district are locked in a bitter feud about how Los Angeles should use the money it already gets.

Although district officials have agreed to come closer to meeting some of the union’s demands, they say fulfilling all of them would bankrupt the system, which is already strained by rising health care and pension costs.

Several Democratic politicians voiced their support for the strike Monday, including Sen. Kamala Harris of California, who is considering a run for president. But Gov. Gavin Newsom, who took office just a week ago, was more measured.

“This impasse is disrupting the lives of too many kids and their families,” he said in a statement. “I strongly urge all parties to go back to the negotiating table and find an immediate path forward that puts kids back into classrooms and provides parents certainty.”

The sprawling district goes far beyond the Los Angeles city limits, stretching some 720 square miles from wealthy coastal areas like Pacific Palisades to working-class southeast suburbs like Montebello. It is overwhelmingly low income; more than 80 percent of students qualify for free or reduced-price lunch. And Latinos make up roughly 75 percent of all students, while whites and African-Americans each account for less than 10 percent of enrollment.

On Monday, we went to schools across the district to talk with teachers on the picket lines, as well as the students and parents who joined them.

Inside Schools, iPads and Crowded Auditoriums

At Robert F. Kennedy Community Schools, a large campus in Koreatown, hundreds of teachers and supporters gathered before sunrise, holding signs declaring “We demand respect” and “Striking for our students.” By 8 a.m., dozens of students and parents had gathered along the sidewalk to support the teachers, but there was also a steady stream of students entering the campus, which houses several small schools.

Sophie Chiang, a 10th-grade student, arrived well before the first bell. “Oh God, it’s really happening,” she said as she approached the line of teachers in red ponchos shouting “Whose schools? Our schools!” The campus security guard who is usually at the entrance was not at his post.

At nearby John Burroughs Middle School, the classrooms mostly sat empty Monday. Roughly 40 percent of the school’s 1,700 students showed up in the morning and were sent to the gymnasium, auditorium and multipurpose rooms to work independently on their school-provided iPads. A school administrator and substitute teacher were stationed in each large space, trying to keep order for hundreds of children.

“We’re trying to make the best learning environment with what we have,” said Steve Martinez, the school’s principal. “It’s not an ideal situation.”

Martinez had asked the district for 65 substitute teachers, but received only six. By lunchtime, the rooms were rowdy but under control — though several iPads and phones clearly displayed games that would not be considered educational.

Still, Matias Garcia, a sixth-grade student, texted his mother asking her to come get him by 11 a.m.

“Nobody is listening, it’s loud and we spent like an hour taking attendance,” Matias said.

His mother, Lilly Santaniello, works part time as a lawyer and hired a relative to come take care of her other sons. Matias said he had a plan for the afternoon: video games and sleep. — JENNIFER MEDINA

Priced Out of the Neighborhood

In a downpour, dozens of teachers, wearing ponchos and waving green and red placards — “We stand with LA teachers” — picketed outside the Paul Revere Charter Middle School in the Pacific Palisades neighborhood. One teacher waved a large American flag, others held bullhorns.

Steven Bilek, a union representative and math and science teacher at Paul Revere, said that for some teachers, the main issue was pay, but others were just as concerned with class size or the lack of support staff like librarians or counselors.

“The idea that teachers just want raises is just not true,” he said.

Paul Revere is a charter school affiliated with the district, and students attend from more than a hundred ZIP codes around Los Angeles, he said. The school is in one of the city’s wealthiest areas, and most if not all of the teachers cannot afford to live nearby.

“I don’t think any of our teachers live in this area,” Bilek said. “We’re lucky to live within an hour.”

Some parents and students joined the teachers on the picket line. “We are here fighting for the future of public education,” said Dennise Weir, whose children, Olivia and Edward, were by her side.

Weir lives in Beverly Hills and can afford private school, but she sends her children to public school because, she said, “they are the last civic institutions that are available to everyone.”

There are about 2,100 students enrolled at Paul Revere, but on Monday about a third showed up for class.

Each grade was gathered in large assemblies, overseen by the few adults on duty — substitute teachers, campus aides and technicians. On a normal day the school has 95 teachers, but on Monday there were only 10 adults with teaching credentials on hand.

— TIM ARANGO

How Does Teaching in LA Stack Up?

Union leaders say teaching in the city is unsustainable, with salaries far outpaced by a high cost of living, large class sizes and not enough resources to help struggling students. Here’s a look at how Los Angeles Unified compares nationally:

— Teacher pay: The average teacher salary in Los Angeles Unified was $75,000 during the 2017-18 school year, according to the California Department of Education — far higher than the national average of around $59,000.

But Los Angeles is an expensive place to live. When you compare the city with other high-priced urban centers, its teacher pay no longer looks so extraordinary. The average salary in San Francisco last year was $73,000, while New York City teachers earned an average salary of $88,000, according to their union.

Beginning salaries matter, too, for attracting and retaining young talent. In Los Angeles, the entry-level teacher salary was $44,000 last year, compared with $47,000 in San Francisco and $57,000 in New York City.

Los Angeles Unified has, so far, offered teachers a 6 percent raise.

— Class size: An independent report noted that the two sides can’t agree on how to calculate class size, but it is clear that classes in Los Angeles are big. The district has offered to cap classes at 35 students in grades 4-6; 39 students in middle- and high-school English and math; and 32 students at elementary schools that serve many low-income children. Nationally, average class sizes in urban schools ranged between 16 and 28 students, depending on grade level and how the school was organized, according to the National Teacher and Principal Survey for 2015-16.

— Professional staff beyond the classroom: One of the union’s main demands is for the district to hire more guidance counselors, nurses and librarians. With increased pressure over the past two decades to raise standardized test scores, many public schools have funneled funds into math and reading instruction, and suffer from a dearth of such professional staff.

California’s situation is worse than the national average, with more than 600 students per counselor across the state and more than 500 per counselor in Los Angeles County, according to an analysis from the Lucile Packard Foundation for Children’s Health. There are nearly 2,000 students per every school nurse in the county.

Los Angeles Unified has offered to add an additional academic counselor to each district high school and to make sure each elementary school has daily nursing services. The district also offered to ensure library services at each middle school.

— DANA GOLDSTEIN

Making Noise

As the final bell sounded at Gerald A. Lawson Academy of the Arts, Mathematics and Science in South Los Angeles on Monday afternoon, striking teachers formed a circle outside the exit and chanted call-and-response songs in the rain. A handful of students lingered with their parents, singing along.

Susie Chavez ducked under a teacher’s umbrella, belting out “We are the students! The mighty, mighty students” with her arms around her children.

Chavez said she would keep bringing them to school no matter how long the strike went on because “the teachers are on strike, not my kids.”

In the car pool line, students in their raincoats packed snugly into Cristina Aguilar’s SUV. While she said she supported the strike, Aguilar wanted to avoid creating a truancy record for her younger children, in kindergarten and third grade. Her eldest had stayed home.

She was taking a wait-and-see approach to school attendance during the strike. “If there’s no teachers, there’s no point in bringing them,” Aguilar said. — LOUIS KEENE

This article originally appeared in The New York Times.