CHICAGO (AP) — A plan to reopen Chicago schools remained in limbo as last-minute negotiations over COVID-19 safety measures with the teachers’ union stretched into Sunday, amplifying the possibility of a strike.

Roughly 62,000 students and about 10,000 teachers and staff in K-8 were expected to start school Monday as part of the district’s gradual reopening, plans Mayor Lori Lightfoot insists will take place. The Chicago Teachers Union has fought returning to classrooms in the nation’s third-largest district, defying orders to come to class ahead of students. The union has said that if the district locks teachers out of email and teaching platforms, which it has done previously, teachers will picket.

Mayor Lori Lightfoot said Sunday night that all-remote learning will continue for Chicago Public Schools students on Monday, but the district plans for some to return to in-person classes on Tuesday. Lightfoot said they’ll push that back by one day as negotiations with the teachers’ union continue.

Lightfoot said teachers are expected to return to schools on Monday whether a deal is reached or not.

“We expect all our teachers who have not received a specific accommodation to come to school tomorrow,” Lightfoot said. “In order to give teachers an opportunity to get themselves ready for students we’re telling parents to take your kids back to school on Tuesday.”

In tweets posted Sunday afternoon, the CTU alleged the district’s bargaining team was told not to negotiate unless the union’s members were willing to offer “major” concessions.

“The CPS bargaining team was instructed not to attend negotiations today unless our teachers, clinicians, PSRPs, nurses, librarians, and other rank-and-file educators were prepared to make major concessions,” the union said in a tweet.

In response, CPS posted to Twitter: “Fact check: Our bargaining team was told by CTU leadership that they were unavailable to meet until they could develop a response to our most recent offer. Our team has been standing by all day.”

Both sides have been fighting for months over issues including vaccinations, metrics used to gauge infections, and special accommodations for employees who have concerns, like a high-risk family member in their household.

Public health officials say there’s growing evidence that children aren’t the main drivers of community spread. They also say school transmission remains low when safety measures, like wearing face coverings, are in place. But debates over reopening have taken place worldwide.

The district’s CEO, Janice Jackson, took to national television Sunday to insist it was safe to reopen Chicago schools with proper protocols. The district, which requires masks for students and teachers, has purchased thousands of classroom air filters, deep cleaned schools, and rolled out a voluntary testing program.

“We believe that we have to reopen schools. We’ve been closed for almost a year now. And as a school system, we’re starting to see some of the effects of schools being closed,” Jackson told CBS’ “Face the Nation.”

District officials said Black and Latino students, who make up the vast majority of the roughly 355,000-student district, have been especially hard hit since going fully remote last March. Pre-K and some special education students returned Jan. 11, but went back to online classes amid the escalating fight with the union. District officials haven’t said when high school students will return.

Union officials say the district hasn’t gone far enough in its safety plans, for instance, by not prioritizing teacher vaccines, and is putting teachers at unnecessary risk. The union also argues that not enough students are interested to require all teachers to come back at once.

About 77,000 students from pre-K to 8 expressed interest in returning to class in a December survey. But attendance has been lower.

Roughly 6,500 of the nearly 17,000 eligible preschool and special education students said they’d like to return, but only about 3,200, or 19% of those eligible, attended after the January reopening, CPS said.

“We’ve been promised repeatedly that CPS’s reopening plans are about equity. But it makes no sense to lockout 100 percent of our students when 80 percent of families are keeping their children remote. That’s the opposite of equity,” the union said in a statement.

The district has said if teachers fail to show up for in-person classes it would amount to an illegal strike. However, the union maintains the district would be forcing the work stoppage by locking out teachers.

The roughly 25,000-member union’s collective bargaining agreement, approved after a 2019 strike, prohibits its members from striking during the terms of the contract and bars district officials from locking out workers.

Illinois recently rolled out its latest vaccine phase that includes residents ages 65 and older and teachers. But districtwide efforts to vaccinate teachers won’t begin until the middle of February. City officials say there are many priority groups and not enough doses.

The Associated Press and NewsNation affiliate WGN contributed to this report.