By Holly Yan, CNN

(CNN) -- After six days and more than $125 million in lost revenue, the Los Angeles teachers' strike is almost over.

Leaders from the United Teachers Los Angeles union and the Los Angeles Unified School District struck a tentative deal Tuesday, ending a heated battle that's left 600,000 students in limbo.

"Pending approval of teachers and (the) board of education, we have an agreement that will allow teachers to go back to work on their campuses tomorrow," Mayor Eric Garcetti announced Tuesday.

The deal was reached after a marathon 21-hour negotiating session that ended around dawn Tuesday morning, Garcetti said.

"It is a historic agreement," said the mayor, who helped mediate the dueling sides. "It gets to lower class sizes. It gets to proper support staff."

LAUSD Superintendent Austin Beutner said the deal includes a 6% raise for teachers and a gradual decrease in class sizes over the next few years. He said more specifics will be released later Tuesday.

While the union's members still need to vote on the deal, UTLA President Alex Caputo-Pearl said he expects the agreement to be approved.

He praised the more than 30,000 teachers and staff members who picketed for six days, often under cold rain.

"I'm so proud of our members, classroom teachers, counselors, nurses, librarians, psychologists," Caputo-Pearl said.

He made those remarks at a news conference standing next to Beutner -- a symbolic sight, considering Beutner has been the target of intense UTLA criticism for months.

While both sides gathered in harmony Tuesday, "the strike was painful, and it had a cost," Garcetti said.

The first week of the strike cost the school district more than $125 million. That's because the state of California funds schools based on daily attendance, and the number of students going to school plunged during the strike.

Some parents said they purposefully kept their kids out of school in hopes that the money drain suffered by the district would lead to a resolution more quickly.

Why so many teachers toughed it out

For the past six school days, educators hit the picket lines to demand smaller class sizes, higher salaries and more school nurses and counselors.

"We work with students every day who face trauma and face hardship," Garfield High School teacher Erika Huerta said. "So we're doing this as a life passion to improve our community."

Teachers and students who joined them on the picket lines decried class sizes of 45 or more. They said many students aren't able to get the individual attention they need to effectively learn.

So the standoff between UTLA and LAUSD came down to two issues: how much money to spend on more school staffing and teachers' raises, and whether the school district actually has that kind of money.

The union wanted the school district to pony up more money to meet its demands, but LAUSD said its $1.8 billion in reserves is already earmarked for education spending during this three-year budget cycle.

The school district agreed that schools need smaller class sizes, more staffing and bigger raises for teachers.

"The issue has always been how do we pay for it," Beutner said. "We can't solve 40 years of underinvestment in public education in just one week, in just one contract."

Firefighters and celebrities join the fight

During their week-long strike, teachers received support from a barrage of actors, musicians and politicians.

On Tuesday, they got a big boost from members of the International Association of Firefighters, who are in Los Angeles for a conference this week.

"We stand with LA teachers," read signs held by dozens of firefighters as they rallied during a parade Tuesday morning.

Some chanted "fighting for justice" as they rode atop a fire truck.

A sign posted on the front of the red truck said "#RedForEd" -- a familiar slogan from the recent wave of teachers' strikes nationwide.