CHICAGO (CBS)– The Chicago Public Schools canceled school again on Thursday – after Chicago Teachers Union representatives voted to accept a tentative agreement, but would not go back to work without the lost days made up.

Mayor Lori Lightfoot said Wednesday night, as she has repeatedly before, that the teachers will not be getting those makeup days.

At a news conference at union headquarters on West Carroll Avenue late Wednesday, CTU President Jesse Sharkey said the union House of Delegates had voted to approve a tentative agreement with CPS.

“We believe this is an agreement that will produce real, lasting benefits in our schools,” Sharkey said.

But because of the makeup day issue, there is no return-to-work agreement in place.

“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 over the past 10 days,” Sharkey said.

All CTU members were instructed to be at City Hall on Thursday at 10 a.m.

Sharkey said if Mayor Lightfoot called him and announced that the days missed due to the strike would be made up, the teachers would return to school on Thursday. But by then, CPS had already declared that school was canceled for Thursday.

CPS held out longer Wednesday night than any other night to declare that school was canceled. The announcement came around 8:45 p.m., and CPS earlier advised it does not make robocalls after 8 p.m.

Sharkey said it was Mayor Lightfoot’s prerogative to wait until September to negotiate the union about class sizes and support staff such as nurses and social workers. But he said now – despite an agreement on those issues that is “not perfect,” but workable – Mayor Lightfoot is trying to stick it to teachers.

“And now, we feel like we’re just being punished because we had the audacity to defy the mayor, and that’s not right,” Sharkey said.

When a reporter asked if she was willing to make up the time if it was unpaid, Sharkey said, “We don’t do unpaid work.”

American Federation of Teachers President Randi Weingarten appeared at a news conference with CTU officials on Wednesday night. Like Sharkey, she said the issue is that students cannot lose class time.

“What I don’t understand – and look, I’m a New Yorker, maybe that’s why I don’t understand it – but what I don’t understand is for a mayor who believes that this instructional time is so important” would not agree to make up the time.

At a separate news conference at City Hall a short time later, Lightfoot’s frustrated tone was palpable as she said she was “gravely disappointed that the CTU has declined to end the strike.”

Lightfoot reiterated that the contract as it stood Wednesday – even as it stood a week earlier – was “the most generous deal that the CPS has ever offered to the CTU.”

“Here we are after students have missed 10 days of class, and the CTU leadership has chosen to throw a curveball into the process rather than say yes to victory,” Lightfoot said.

Mayor Lightfoot said when Sharkey and Gates met with her for three and a half hours at City Hall on Wednesday, Sharkey identified four issues that he said were still sticking points and Gates offered two more.

The mayor said Sharkey told her, “Mayor, I give you my word that these six issues are the last issues that we need to resolve in order for our contract to be ratified.”

But now, Lightfoot said, the CTU has gone against that promise.

“Not once during that three-and-a-half-hour meeting did they raise compensation for strike days. Not once. The issue never came up,” Lightfoot said.

As to the union’s demand for makeup days, Lightfoot said the union would not be getting it.

“I’ve been clear from day one that CPS will not make up any strike days, and at this late hour, we are not adding any new issues,” she said.

Unlike Sharkey and the CTU, Lightfoot did not characterize the issue as one of kids needing the time in class.

“Where we are right now is I’m not compensating them for days they were out on the strike. I’m not going to negotiate,” Lightfoot said. “We gave them a deal on the table they said these were the issues, we met them where they were and that’s it.”

RELATED: Chicago Teachers’ Strike Is 7th Of Past 40 Years

State law does require CPS to make up three of what will be at least 11 missed days — though not necessarily in the way the union has in mind. 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.

But Matthews added that the days may be made up anytime during or at the end of the school year, expect for state-mandated holidays. All Illinois school district calendars have five emergency make-up days built in as it is.

The issue is that CTU wants all the days they will have been on strike tacked onto the end of the school year. Teachers would be paid for those extra days, Molina reported.

Mayor Lightfoot has repeatedly said that idea is a no-go.

CPS said Tuesday that it was evaluating options for making up the three days required under state law.

“The district is in the process of gathering a full understanding of potential outcomes and next steps regarding whether or not the district will make up school days missed beyond eight,” CPS said in a statement issued Tuesday. “The Board would have to vote to add on any additional student attendance days and the district hopes to have additional information and a decision prior to the November Board meeting.”

Meanwhile, despite the strike continuing, CTU officials seemed pleased with the contract. The contract has class-size enforcement – something that a teachers’ contract has never had before, said CTU Vice President Stacy Davis Gates.

But the House of Delegates’ vote on the agreement Wednesday night was far from unanimous. And of course, despite the vote to approve, the longest Chicago teachers’ walkout since 1987 continues.

The latest logjam about makeup days was evident on Wednesday afternoon before the House of Delegates vote. Gates said earlier in the day that the union “may have reached a monumental agreement and want to convene our (House of Delegates) to suspend the strike.”

But at the same time, Gates criticized Mayor Lori Lightfoot for “reducing instructional time,” that is, not making up school days lost by the strike.

CBS 2 Political Investigator Dana Kozlov learned that delegates reached out to teachers on Tuesday, asking them to review the latest proposal for their consideration. That was in anticipation of a possible vote on a tentative agreement Wednesday night.

As CBS 2’s Mike Puccinelli reported, talks between the CTU and CPS ended for the day late Wednesday afternoon at Malcolm X College.

Sticking points also recently included the length of the teachers’ contract, paid prep time for grade school teachers, and class size. But an agreement on those issues was found in the tentative deal.

On Tuesday, talks between the CTU and CPS ended with no deal – after what also turned out to be a false alarm about the strike possibly ending.

That day, the CTU rank-and-file accused Mayor Lightfoot and CPS on Tuesday of misleading the city by suggesting the House of Delegates had met for a vote – when in fact the Tuesday meeting was only to let its members know where they stand.

The union accused the mayor and CPS of sending a robocall to parents claiming the House of Delegates might vote to accept a tentative contract agreement, when she knew there was no deal to approve.

With the strike still going on, so did pickets and protests. Earlier in the day Wednesday, dozens of teachers took their protest to the University of Illinois at Chicago campus. Police stood guard as they chanted near Roosevelt Road and Halsted Street.

Students joined the protesting teachers to lend their support.

Meanwhile on Wednesday, CPS announced it would allow high school football teams to resume practicing during the teachers’ strike, so the teams can meet requirements to qualify for the state playoffs if the strike ends in time for the postseason.

The Illinois High School Association requires football teams to have three practices before they can play a game if the team hasn’t practiced in seven days, and CPS teams haven’t practiced since the strike began on Oct. 17.

It appeared all 19 CPS schools that are playoff eligible were arranging practice Wednesday afternoon or evening so they could get all three required practices before Saturday.

But the strike still would have to end before the playoffs start on Saturday in order for the CPS teams to play. If the strike doesn’t end by the playoffs, all CPS teams would forfeit their games. Although the state playoffs begin Friday night, no CPS teams have games scheduled until Saturday, so it’s unclear exactly when the strike must end for them to play.