On the 10th day that Chicago students were out of class because of the Chicago teachers’ union strike, the educators announced that they had reached a "tentative agreement" but do not have a return-to-work agreement.

The Chicago Teachers Union said in a tweet Wednesday that they would be at City Hall on Thursday to demand that Mayor Lori Lightfoot commit to making up school days lost to the strike.

The union's House of Delegates approved the tentative labor agreement, but delegates have been clear they won't go back to work unless there's a provision to make up the instructional days, the union president said Wednesday night. The vote was approved, 362-242, NBC Chicago reported.

The mayor has refused to add makeup dates to the end of the school calendar, NBC Chicago reported. According to union materials, teachers are not paid for school days spent on strike, The Associated Press reported.

Lightfoot said at a Wednesday night news conference that she was "gravely disappointed" that the strike would continue for an 11th school day.

"I'm not compensating them for days that they were out on strike — I've been very clear about that from day one," Lightfoot said. She accused the union of moving the goal posts during negotiations.

The mayor said the city has offered the union a "historic deal, by any measure" that includes compensation, class size, staffing, resources for the homeless and other issues. "The contract as it stands today, even as it stood a week ago, is the most generous deal that CPS has ever offered to the CTU," she said.

The nation's third-largest school district canceled classes after the teachers' union went on strike earlier this month.

We have a tentative agreement, but we do not have a return to work agreement. So we will be at City Hall at 10 a.m. to demand the mayor return our days. — ChicagoTeachersUnion (@CTULocal1) October 31, 2019

CTU President Jesse Sharkey said at a news conference Wednesday night that teachers are "happy about " the tentative agreement but that "we're not walking away from two weeks of school."

"We do not understand why the mayor cannot simply call and say, 'We'll give you an agreement to make up the instructional time,'" Sharkey said. "If the mayor calls, and I take that call and she says that we have an agreement on that, we'll be back at work tomorrow."

Sharkey said that no one's asking to be paid to strike, and "we want to teach our students."

He said commitments about school nurses, class size, social workers and resources for homeless students wouldn’t have been possible if teachers didn't walk picket lines. Any contract would need to be ratified by union members.

The union voted to authorize a strike and teachers and others went on picket lines Oct. 17 after what the union said was 10 months of "frustrated" bargaining. The strike also complicated life for some parents who have said that because students are not in school, they've had to find babysitters or make other arrangements. But others have expressed their support.

Although classes were canceled, school buildings remained open to provide students a safe place to go during the day, and meals were being served, the mayor has said.

The union House of Delegates voted Wednesday to accept the tentative agreement, 362-242, NBC Chicago reported.

The last time the Chicago Teachers Union picketed was in 2012 and it lasted seven days. The district narrowly avoided a strike in 2016, after the union came to an agreement with then-Mayor Rahm Emanuel.