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LOS ANGELES — Teachers in the nation's second-largest school district are expected to strike for a sixth school day Tuesday as talks between Los Angeles Unified and United Teachers Los Angeles continue.

The union said Monday that teachers are due back at picket lines Tuesday morning even if an agreement is reached Monday, saying it takes time to mobilize a ratification vote of a deal.

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Los Angeles Mayor Eric Garcetti remained upbeat as he mediated the fifth day of marathon negotiations at City Hall on Monday.

"The parties are still at the table, and I am optimistic that we have the momentum to take those final steps toward bringing our teachers and young people back into their classrooms," he said in a statement.

Tens of thousands of educators walked off the job and onto picket lines Jan. 14 for the first time in 30 years. The union and LA Unified School District are at odds over issues including salary, class size and support staff.

Teachers have complained about overcrowded classrooms where students sit on window sills or on the floor.

The district has about 600,000 students in K-12. Schools have stayed open during the strike with a skeleton staff.

More than 1,000 firefighters from across the U.S. and Canada who are in Los Angeles as part of an international conference are scheduled to march on behalf of striking educators on Tuesday.