Chicago Public Schools announced Sunday that classes and after-school activities will be canceled again Monday after a frustrated Mayor Lori Lightfoot and district officials said negotiations with striking teachers remained stalled.

CPS CEO Janice Jackson joined Lightfoot in a rare Sunday night news conference in which the mayor blasted the Chicago Teachers Union over the impasse in contract talks.

“We are enormously disappointed that CTU cannot simply take yes for an answer,” Lightfoot said.

“As of today, we’ve put everything we could — responsibly — on the table in an attempt to get a deal done, but we have no deal to announce today,” Lightfoot said. “For that, I am terribly disappointed.”

The labor dispute that involved members of the Chicago Teachers Union and SEIU Local 73 has now prompted the cancellation of eight days of classes for CPS’ 300,000 students, surpassing the bitter 2012 strike to become the city’s longest schools walkout since 1987.

But SEIU Local 73 reached a tentative agreement Sunday night with the city even as CTU talks apparently slowed.

Lightfoot said the city put forth the “most generous offer in CPS history” to the CTU, which included raises for teachers and staff and addressed class sizes and support staffing by prioritizing schools with the greatest need.

“CTU have told us, told their membership and told the entire public that they wanted to get this contract to transform public education in Chicago, and this is exactly what our offer does,” Lightfoot said.

Under the city’s offer, the average Chicago teacher will see a $100,000 salary by the end of the contract’s lifespan, Lightfoot said.

Jackson echoed Lightfoot’s frustrations with the union, saying that the city has met each of their demands and put it in writing as CTU requested.

“There’s no magic pill to fix this overnight, and there’s certainly no billion dollars in our state waiting to cover these expenses,” Jackson said.

Jackson claimed that CTU leadership’s remaining demands would require another $100 million in funding on top of the city’s current $500 million offer, ”which the district can simply not afford.”

But at their own press conference later Sunday held outside Malcolm X College where bargaining has been taking place, CTU officials disputed the gap was so large.

CTU Vice President Stacy Davis Gates said $38 million — .5% of CPS’ annual budget — separates both parties at the negotiating table.

When asked whether the city was misrepresenting the figures, the entire CTU bargaining team gave an emphatic “yes.”

She said that the city’s offer on class sizes would impact just one-third of the city.

”That’s an achievement, but that’s not enough,” Davis Gates said.

On staffing, that city said it has agreed to place a full-time social worker and nurse in every school, but Davis Gates said that most schools will be without librarians under the city’s offer.

But Davis Gates said the “most egregious gap” between the city and the union was on the issue of special education.

”So when I hear there’s a deal on the table and we can’t say yes, I have to ask: Do we say yes to injustice [or] inequity? Do we say yes to things that are half-done?” Davis Gates said.

”I need the parents in this city to know that everything we’re stuck on in this moment is about the quality of education in Chicago Public Schools,” she said.

Another stumbling point is the teachers union is seeking a three-year deal, while the city has asked for five years. CTU President Jesse Sharkey made it clear that the union would only consider the longer contract if the city agreed to additional money to address teacher preparation time and class size, which he said wasn’t addressed in its latest offer.

In an earlier statement, he said “one of the richest cities ... in the world” should be able to find the money to settle the contract.

“Their misplaced priorities will put us on the picket lines again tomorrow,” he said.

‘Stay in the fight’

At a solidarity rally Sunday afternoon at New Mt. Pilgrim MB Church in West Garfield Park, CTU members and their supporters prayed, danced and sang to encourage CPS teachers and support staff to keep fighting for what they believe will be a fair contract.

“Don’t let anybody tell you that kids need to be in their classroom. We all know that, so you stay where you are,” said Christel Williams-Hayes, the union’s recording secretary.

Between speakers, a live church choir and band led supporters in singing uplifting songs to motivate the striking teachers and support staff.

“Don’t give up. Don’t give in. Keep on fighting. Stay in the fight,” the choir sang while the audience rose from their pews to dance, praise and sing along.

National civil rights leader Rev. William Barber II, who was scheduled to attend the rally but couldn’t due to travel complications, Skyped in.

Chicago City Treasurer Melissa Conyears-Ervin, who has joined teachers in several rallies and picket lines since the strike began, spoke about her experience growing up in Englewood and the West Side as a Chicago Public Schools student.

Conyears-Ervin said she and her two sisters were raised by a single mother who relied on CPS for their education.

“It was the public schools system that helped get me where I am today,” she said. “So I refuse to accept that our children should have to choose between a teacher, social worker, nurse or a librarian. I demand and decree that our children deserve nothing less than the best.”

Other speakers included Cook County Clerk Karen Yarbrough and Cook County Board Commissioner Brandon Johnson.

Loretta Johnson, secretary-treasurer of the American Federation of Teachers, said that schools in Baltimore are facing the same issues — growing class sizes, low wages and a lack of support staff — that CTU is working to address. She encouraged Chicago teachers to keep fighting for “educational justice.”

“You are on strike because you want what’s best for students. Don’t let nobody turn you around,” Johnson said. “All across the country, we’re watching [because] you are standing up and fighting for all of us.”