Tens of thousands of teachers in the nation's second-largest public school district took to the streets on Monday to rally for smaller classes, higher salaries and more support staff on the first day of a strike in a district with 600,000 students.

The walkout, the first in city schools in almost 30 years, took place after months of failed negotiations between the United Teachers of Los Angeles (UTLA) and the Los Angeles Unified School District (LAUSD).

"We think it's unfair that they keep on giving us more students, more work, different caseloads, students with different needs in the regular classrooms, and we don't have enough support staff," said Janine Kish, a science teacher at Southeast Middle School who marched with several of her co-workers.

She said some schools have a nurse on campus only one day per week, which prevents children from getting immediate help.

"The money is there; we need to invest it on our students. We spend $71,000 per jail inmate a year, but how much do we spend on each student per year?" she asked.

Yolanda Sandoval, a teacher at Fourth Street Elementary School in East Los Angeles, said the issue that most concerned her was class size, which hasn't met the California norm for a long time, she said.

"We want our students to be able to get all the attention that they need, but that's the basic thing," she said, adding that the number of pupils in Grades 4-6 is in the mid- to high 30s.

According to the California Department of Education, the average number of students per teacher in Grades 4-8 should not exceed 29.9 or the district's average number per teacher in 1964.

The LAUSD served 600,000 students across more than 1,000 schools in kindergarten through Grade 12. The district also has more than 200 independently operated public charter schools.

Chanting, "we are the union, the mighty, mighty, union," the teachers, who congregated in front of Los Angeles City Hall, marched 1.3 miles to the Los Angeles Unified School District.

Although rainy, chilly weather forced them to huddle under umbrellas, the protesters proudly donned red T-shirts — the color that stands for the teachers' labor movement, and held plastic homemade signs that read "We stand with the LA teachers" and "On strike for our students."

UTLA President Alex Caputo-Pearl rejected the latest offer from the school district Friday, calling it "inadequate".

That offer included a 6 percent raise and back pay to the 2017-18 school year, in addition to a reduction in class size, 1,200 more educators in schools and a $130 million investment for 2019-20.

"Over the weekend, we've heard nothing more from the district. We are in a battle for the soul of public education," he said during a pre-strike news conference.

LAUSD Superintendent Austin Beutner said Monday that he urged the union to resume bargaining with the district, "anytime, anywhere, 24/7".

"We are in discussions with the governor, with the mayor, with the state's superintendent of education, because it's our desire to have all the educators well-supported and back in school serving the needs of our students," he said.

Mayor Eric Garcetti told reporters Monday that he's working with both sides to come up with a resolution.

California Governor Gavin Newsom said in a news release Monday that last week he submitted "a budget to the Legislature that would make the largest ever investment in K-12 education, help pay down billions in school district pension debt and provide substantial new funding for special education and early education".

The union said on its website that it has been negotiating with the district since April 2017, and that teachers have been working without a contract for almost a year.

"LAUSD refuses to negotiate a fair agreement that addresses our critical needs, even though they are sitting on $1.8 billion in reserves," the union said.

The district, however, said that it does not have as much money as the union believes.

In 1989, Los Angeles teachers walked out in a strike that lasted nine days.

Schools were open Monday despite the strike. The district said it was hiring 400 substitute teachers and directing 2,000 district staff members who hold teaching credentials to replace the striking teachers.

But Euphronia Awakuni, whose daughter is a second-grade pupil at Castelar Elementary School in Chinatown, which provides a Mandarin language-immersion program for its students, said she is worried about the length of the strike.

"I am lucky because I don't have to work this month, so I can take the kids and help out, but it's scary, because the federal government doesn't work, and the teachers are on a strike," she said.

Asked if she's worried that the strike will continue indefinitely, Kish said the job action "sends a voice to the right people that we are not willing to take it".

"We are not here just for our rights, but the students'," she added.

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