by Todd Feurer, CBS Chicago web producer

CHICAGO (CBS) — Mayor Lori Lightfoot said Thursday she’s willing to negotiate with the Chicago Teachers Union on how many lost days of school will be made up for the days spent on strike, but she won’t agree to the union’s “take it or leave it demand” to compensate teachers for all 11 days that have been missed so far.

“We have been told by the CTU that they will end the strike only if we agree to make up all the days missed due to the strike. They have basically issued a take it or leave it demand. They get 11 days back or the strike continues. That’s simply a non-starter,” Lightfoot said Thursday morning at City Hall. “Both sides must have a spirit of compromise. That’s what the CPS bargaining team brought to the table every single day.”

The mayor said she’s willing to have a conversation with the CTU about how many lost days of class the district will make up after the strike ends, but won’t cave to an all-or-nothing ultimatum.

On Wednesday, the union’s House of Delegates voted to accept a tentative contract agreement with the Chicago Public Schools, but did not end the strike, demanding CPS agree to make up for every day of class that has been missed.

RELATED: Highlights Of The Tentative Agreement

Teachers have been on strike since Oct. 17, and students have missed 11 days of class so far.

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The mayor noted she said even before the strike started that she did not plan on making up for lost days of class, and the union didn’t demand makeup days until they had reached a tentative contract agreement.

Lightfoot said in a meeting with union leaders on Tuesday, they told her the issues that would need to be addressed in order to end the strike, and making up for lost class days was not mentioned.

“Wouldn’t you think if this was a priority, that they would have brought it to the table?” she said. “They didn’t do that, and we are where we are now, and if they’re willing to come to the table in a spirit of compromise, I’m certainly willing to listen, but this is on them.”

Later Thursday morning, as teachers rallied outside the Thompson Center across the street from City Hall, CTU President Jesse Sharkey said he was confident the union could reach an agreement with the mayor if she’s willing to talk.

“I understand that it’s going to be difficult for them mayor and for the people that run the schools to fit 11 instructional days back into the school year, but our strong feeling is that we want to discuss it. It can’t be zero. It can’t be zero. That’s not an acceptable number to us,” Sharkey said.

Sharkey said he planned to go to City Hall to try to meet with the mayor on Thursday.

“I’ll discuss it, and we’ll find a number that we can make a compromise on,” he said.

Sharkey said the mayor should have known the union would want to make up for lost school days before agreeing to return to work. He said a “return to work agreement” is a standard part of teachers’ contract negotiations, and that making up for at least some lost school days is part of those agreements.

“Just because the mayor doesn’t know how teachers’ negotiations work, I don’t know what to say about that,” he said. “Her attorney knows it full well, because she has an attorney who’s been negotiating this stuff since the mid 1980s.”

Illinois State Board of Education spokeswoman Jackie Matthews told CBS 2’s Tara Molina that statute requires 180 school days per school year – counting teacher institute and parent-teacher conference days when students may not be in school.

CPS has 178 days when actual students are in school, along with six days for teacher workshops or parent-teacher conferences. This means CPS’ calendar officially has eight more school days than are required by state law. But Thursday will make 11 days, and thus, three will have to be made up, Matthews said.

However, Mayor Lightfoot disputed that, saying ISBE told her the city won’t have to make up for ay lost days of school so far.

Lightfoot said Sharkey never brought up compensating teachers for lost days due to the strike as part of its demands for a new contract.

“We cannot allow the CTU leadership to continue to make repeated new demands, and move the goalposts unilaterally and repeatedly,” Lightfoot said.

After the mayor’s press conference at City Hall, hundreds of teachers rallied outside to call on Lightfoot to agree to their demand for makeup days.

“Our delegates told us in no uncertain terms that we are not going back to work unless there is a provision made for making up the instructional days that have been lost,” Sharkey said Thursday night.

CPS Chief Executive Officer Janice Jackson said the only way to make up for all 11 days of missed classes would be to either extend the school year until the end of June, or to cancel winter break, when many parents already have made plans for those days off of school.

“This year has already been disrupted because of the strike,” Jackson said.