(CNN) Chicago public schools students will miss classes for a fourth day on Tuesday as a teachers' strike drags on.

The Chicago Teachers Union and city officials negotiated through the weekend but couldn't reach a deal. On Monday, about 10 different proposals on various issues went back and forth, a senior city official told CNN. The Chicago Teachers Union enlisted the help of the Rev. Jesse Jackson, who attended a bargaining session, to mediate.

A Democratic presidential candidate will add her voice in support of the teachers. Massachusetts Sen. Elizabeth Warren is set to join Randi Weingarten, president of the American Federation of Teachers union, and educators on the picket lines on Tuesday, the union said. Bargaining resumes Tuesday, the Chicago Teachers Union said.

Chicago Teachers Union members gave Mayor Lori Lightfoot a grade of 'F' for the latest round of negotiations, union president Jesse Sharkey told reporters Monday night.

"The mayor has dashed our hopes for a quick settlement," Sharkey, said citing a letter Lightfoot sent to the union urging them to end the strike as negotiations continue.

"We are not going back to work without a legally binding agreement," Sharkey said.

Lightfoot's letter, which was shared on the union's Twitter account , said in part: "The students and families of Chicago cannot afford to be out of school for any longer, which is why we are asking you to end the strike and encourage your members to return to work while bargaining continues."

"Our team has been turning around thoughtful counteroffers at a rapid pace. We are hopeful that CTU will meet that pace ... so we can bring this process to a fair and responsible end."

The union's demands echo what teachers across the country are fighting for : smaller class sizes, more support staff, higher raises and more school funding.

But the mayor and Chicago Public Schools say it's just not realistic to fund everything the union wants.

"CPS is not flush with cash," the mayor said. "The fact is there is no more money. Period."

'Tragedies waiting to happen'

Chicago teachers say they're fighting for students who often face dire challenges

About 75% of their students qualify for free or reduced lunch. In some neighborhoods, gangs and violence permeate the streets, forcing children to grapple with grief at a very young age

Nine of 10 majority-black schools have no librarians, and there aren't enough bilingual teachers in a district that's "nearly half Latinx," the union said.

My sons understand how important education is. They have walked the picket line with me & their teachers in 2012 & 2019- I'm a proud CPS mom/teacher- we are fighting for all of the children in CPS to get the same advantages regardless of their zipcode pic.twitter.com/y8kaykrLPZ — Ms. Middle School Math Teacher (@AlisonH38184157) October 20, 2019

And many schools don't have a full-time nurse.

"There (are) just tragedies waiting to happen because we don't have enough staff in our schools," nurse Dennis Kosuth told CNN affiliate WLS

Last year, Kosuth had to split his time among six schools.

"It was impossible for me to give the kind of care that I wanted to give to my students," he said.

This year, he's working at three different schools. "But I'm still just as busy."

Hiring more social workers, counselors, nurses, bilingual teachers and librarians is just part of of the union's demands.

Teachers also want smaller class sizes, higher pay for all school employees and more teacher prep time during the school day.

More than 41,000 Chicago elementary school students are trying to learn in classes with 30 students or more, the union said. Of those, 5,290 are in classes with at least 35 students.

And from the elementary to high school levels, CTU said, some classes have more than 40 students.

Chicago kids need far smaller & enforceable class size limits for an equitable and quality education. @classsizematter #classsizematters cc @chicagosmayor @CTULocal1 pic.twitter.com/H4QCrU2q21 — leonie haimson (@leoniehaimson) October 20, 2019

A bit of progress

Chicago Public Schools has offered to steadily raise teachers' salaries to an average of $97,757 by fiscal year 2024

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"We will also ensure every school has a full-time nurse by 2024," CPS said.

It said it would also commit another 200 social workers and special education case managers for the highest-need schools over the next three years.

But the challenge isn't just funding those new positions -- it's finding enough quality applicants.

"Social workers, nurses, counselors, and other similar positions are hard to hire," the school district said. "The candidate pool is limited, and hiring is competitive."

Last month, the union asked for "CPS to hire more than 1,000 new employees by October 1, 2019, across several hard-to-find specialties," the school district said.

"The (union's) proposal also calls for hiring approximately 3,000 more employees over the next two years at a cost of more than $800 million. Even if CPS could realistically afford such a commitment, it would be nearly impossible to meet those hiring goals."

After lengthy negotiations Saturday, the teachers' union said both sides are getting closer to an agreement -- but sticking points remain.

"We have tentative agreements on eight different items -- two in particular, I think, are huge," CTU Vice President Stacy Davis Gates said.

"One goes with the pipeline for teachers of color," which could help reverse "the precipitous decline of black teachers," she said.

"The other one that's huge is that over the life of the contract, we effectively have a charter (school) moratorium."

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While many parents have joined teachers on the picket lines, some oppose the strike.

"To me, it's a whole distraction and interruption to the school year," said Liam Boyd, the father of a fourth grader at Blaine Elementary School.

"Give them the 16% (raise). I'm fine with that. And I understood we could do with more resources to support teachers in school," he said.

But "I don't support the union. I think the school district and the city has been more fair this time and (are) trying to be more fair."

The union's president, Jesse Sharkey, said he has two children in the school district.

"We understand that a strike is a disruption to the parents of the city," Sharkey said. "It's worth a short-term disruption if that puts in place over the long-term the conditions that make education better in this city."