Months of slow-moving negotiations, back-and-forth attacks and public posturing between Chicago officials and the city’s teachers union have led to 32,500 teachers and school support staff deciding to walk off their jobs and onto picket lines Thursday morning.

The teachers strike, the city’s first in seven years and only the second in more than three decades, leaves parents scrambling to find child care and puts nearly 300,000 public schools students out of classrooms for the foreseeable future as the two sides keep trying to work out a deal.

The decision to forge ahead with a work stoppage was finalized Wednesday evening by the Chicago Teachers Union’s House of Delegates, its governing body which turned down the city’s current offer and ensured an extension of the months-long saga.

Union president Jesse Sharkey said “we’ve got a ways to go” to agree to a deal.

“We have not achieved what we need to bring justice and high-quality schools to the children and teachers of Chicago,” Sharkey said at the union’s Near West Side headquarters. Asked how long the strike would last, he said the union hoped for a short strike but that it would be “up to the mayor.”

An exasperated Lightfoot, meanwhile, said she was disappointed the CTU was carrying on with the strike and wasn’t sure what else she could do to meet the union’s demands.

“At every turn, we’ve bent over backwards to meet the union’s needs and deliver a contract that reflects our shared values and vision for our schools and the support of our students,” Lightfoot told reporters at City Hall Wednesday morning.

The union’s decision to move ahead with the walkout came hours after, in a preemptive move, Lightfoot and CPS officials canceled classes for Thursday in response to Sharkey’s message the night prior that he was “overwhelmingly certain” the union would strike.

Though officials said they made the early announcement to give parents enough advance warning, the move surprised teachers and students, who first heard of the cancellation through news media reports while at school. Some teachers questioned whether the move meant the district was locking out union members or placing them on furlough, but a CPS spokesman said that wasn’t the case.

Parents later received a robocall with a recorded message from schools chief Janice Jackson, who detailed the school district’s vast contingency plan that was set in motion by the stoppage.

Every public school in the city will remain open Thursday with lunch, breakfast and a take-home dinner available for students. Regular bus service won’t be available, but the Chicago Transit Authority is offering free train and bus rides to all CPS students for the duration of the strike.

Dozens of other community centers, churches and public spaces will be open to students. Chicago Park District buildings will also be available after 2,500 union members agreed to a new labor deal with the parks system on Wednesday, a day before they were set to join school workers on the picket line.

“This is an all-hands-on-deck effort. Make no mistake about it,” the mayor told reporters Wednesday evening after the strike became official. Lightfoot assured parents that “if students go to their school, they’re going to see a familiar and friendly face, so they should have no concerns about safety issues.”

Another 7,500 school support staff workers represented by Service Employees International Union Local 73 — including special education classroom assistants, custodians, bus aides and security guards — also said their contract talks broke down, and they will walk out with teachers.

“There is no reason why in Chicago, a city of such wealth and resources can’t properly invest in good jobs for school staff,” said Dian Palmer, SEIU Local 73 president.

Bargaining continues Thursday

Bargaining is set to resume Thursday at a neutral location, unlike previous sessions that alternated between CPS and CTU turf. Although talks continuing despite the walkout is encouraging, the two sides didn’t appear close to a deal heading into the work stoppage.

Chief among the reasons why Lightfoot believed a strike should have been averted is the city’s offer of a 16% pay raise over five years. The mayor has called that a “generous offer that honors teachers.” The union has said it wants a 15% increase in a three-year deal.

Taking more prominence at negotiations, however, have been the CTU’s demands for the mayor to put in writing guarantees for lower class sizes and increased staffing of nurses, librarians, social workers and counselors.

Going into the weekend, the city and union remained apart on most issues, but there were signs of progress after bargaining on Saturday, when both sides toned down their rhetoric and signaled that a path to a deal was becoming clearer. Lightfoot, for the first time and after months of holding out, said she was willing to draft contract language on those two key concerns.

Within 48 hours, however, those discussions appeared to fall apart.

The CTU accused the mayor of reneging on her sudden promise to put those demands in writing, saying her messages in public have not mirrored her negotiating team’s stance at the table.

The mayor, meanwhile, said the union held up negotiations by spending too much time on ancillary demands, leaving too many issues left to resolve with too little time to find solutions.

The strike is Lightfoot’s first big hurdle as mayor and promises to test her resolve just months into her term. Lightfoot had hoped to avoid a strike similar to the one that came early in her predecessor Rahm Emanuel’s time in office, a stoppage that turned into an embarrassment and a black eye on the former mayor’s resume.

$2.5 billion in costs

Earlier Wednesday, the mayor said it wouldn’t be fiscally responsible of her to accede to demands that she estimates would add $2.5 billion to the school district’s budget.

“We value the workers,” Lightfoot told reporters at City Hall. “But I also must be responsible for the taxpayers who pay for everything that goes on.”

School board President Miguel del Valle joined Lightfoot at the news conference and said he believed the CTU had “stopped bargaining in good faith.”

Del Valle said he agreed with the CTU’s goals on staffing and class size, and he believed the mayor had met the union’s demands on those issues.

“The Chicago Teachers Union was insistent ... that it be in writing,” del Valle said. “The mayor responded. What’s left? I am very disappointed that the Chicago Teachers Union has decided that the common good, which I agree with, is their reason for a work stoppage.”

Christopher Hallom, a special ed classroom assistant (SECA), said his 8-year-old daughter Alaysia, will be joining him on the picket line at Perkins Bass Elementary School in Englewood Friday.

“I’m double duty as a parent and SECA, so I understand both sides,” he said. “You want what’s best for the kids and for them not to miss out of classes.”

Hallom said when no deal was reached Tuesday he knew that they would be striking. It’s his first.

“I got a lot of adrenaline. Not knowing one way or the other. It’s been a lot of anticipation,” he said.

If the strike continues for days, he said he’d rely on family to help watch his kid, who said she was looking forward to sleeping in.

Contributing: Jake Wittich