Chicago parents and community groups are scrambling to prepare for a massive teachers’ strike set to begin Thursday, prompting the city to preemptively cancel classes in the nation’s third-largest school district.

The Chicago Teachers Union confirmed Wednesday night that its 25,000 members would not return to their classrooms Thursday after months of negotiation between the union and Chicago Public Schools failed to resolve disputes over pay and benefits, class size and teacher preparation time.

The strike is Chicago’s first major walkout by teachers since 2012 and city officials announced early Wednesday that all classes had been canceled for Thursday in hopes of giving more planning time to 300,000 students’ families.

“We want this to be a short strike with an agreement that will benefit our schools and our teachers. We have a ways to go,” Chicago Teachers Union President Jesse Sharkey said during a union news conference. “We actually want to see improvement on all the issues we are talking about here.”

Mayor Lori Lightfoot said she was disappointed by the union’s decision to strike.

“We are offering a historic package on the core issues — salary, staffing and class size,” she said Wednesday night at her own news conference, adding that school district negotiators will remain at the bargaining table and that she hopes the union does, too.

During the 2012 strike, the district kept some schools open for half days during a seven-day walkout. District officials said this time they will keep all buildings open during school hours, staffed by principals and employees who usually work in administrative roles.

Breakfast and lunch will be served, but all after-school activities and school buses are suspended in the district serving more than 300,000 students.

Janice Jackson, the district’s CEO, encouraged parents to send their children to the school that they normally attend, however they will be welcome in any district schools.

“We’ve put together a really comprehensive plan for the students,” Jackson said. “We will make sure they are safe and they have a productive day.”

Also striking will be 7,000 support staffers, whose union also failed to reach a contract agreement.

Before the strike announcement, June Davis said if teachers walked out, she would likely send her 7-year-old son, Joshua, to his usual elementary school — Smyth Elementary on the city’s South Side where almost all students are low-income and minority.

Davis, 38, said she would otherwise have to take her son to his grandmother’s in a southern suburb, requiring an hourlong trip on a regional bus line.

“Everybody’s hoping they will come to some kind of agreement, find some compromise,” Davis said.

Lightfoot preemptively announced that classes on Thursday would be canceled, saying she wanted to give parents more time to plan. A clearly frustrated Lightfoot said the city had not only offered a 16% pay raise over the five-year contract, but the city also had agreed to put language in the contract addressing “enforceable targets” on class size and increasing staffing levels for positions such as nurses, librarians and social workers — items the union said were critical.

She said the union’s demands would cost an unaffordable $2.5 billion per year.

Union leaders disputed Lightfoot’s characterization of the city’s willingness to concede to their demands on several issues, including class sizes.

“CPS’ current class size offer falls far short of what’s needed to address the sweeping scale of the problem,” they said in a statement.

Lightfoot said the city agreed to make substantial changes on some of the union’s top priorities, but its negotiators responded by issuing additional demands, including some she deemed unacceptable.

“The union is still demanding to shorten instructional time by 30 minutes in the morning,” she said. “We won’t do that. We will not cheat our children out of instructional time.”

Before heading into a downtown law firm for bargaining talks Wednesday morning, union vice president Stacy Davis Gates said there is a “gross disconnect” between Lightfoot’s comments and what negotiators have put in writing.

“To say that you have offered a proposal that respects what we are asking for, to say you’ve bent over backward … it’s absolutely ridiculous,” Davis Gates said.

Community organizations have been preparing for days to welcome students, ranging from a $100 per day camp for elementary school kids at the Shedd Aquarium to all-day programs run by the Boys & Girls Club of Chicago and accessible for a $20 annual membership fee.

Mimi LeClair, president of the Boys & Girls Club of Chicago, said a strike is particularly difficult for single parents and those whose jobs have inflexible schedules.

“It’s a horrendous dilemma, deciding between likely losing their job or having their paycheck docked when they rely on every penny or leaving their children home alone,” LeClair said.

The city’s public libraries also are planning programs for students, along with a network of churches and community centers that are part of the city’s Safe Haven program intended to give kids a safe place during the summer months particularly on the city’s South and West sides.

The YMCA of Metro Chicago expects highest demand for its all-day programs for children between the ages of 5 and 12, who are too young to stay home alone but whose parents may oppose sending them to schools unstaffed by teachers.

“Real life still happens,” said Man-Yee Lee, a spokeswoman for the organization. “Parents still need to go to work and their kids still need somewhere to go.”

(AP)