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CHICAGO (AP) — Leaders of Chicago Public Schools have canceled classes Wednesday after the teachers union voted to switch to remote learning due to record COVID-19 levels. The move comes amid an escalating battle over pandemic safety protocols in schools. The status of instruction the rest of the week remained in limbo.

THIS IS A BREAKING NEWS UPDATE. AP’s earlier story follows below.

Classes in the nation’s third-largest school district will be canceled Wednesday if the teachers union votes to switch to remote learning due to the latest COVID-19 surge, the head of Chicago Public Schools said Tuesday.

Schools CEO Pedro Martinez’s comments come as the district and Chicago Teachers Union are locked in an escalating battle over safety protocols. Students returned to class Monday after a two-week winter break with COVID-19 infections and hospitalizations fueled by the omicron variant at record levels. Voting by union leadership and members was scheduled to end Tuesday evening.

District officials have said they won’t switch back to remote instruction districtwide because it was devastating for children’s learning and mental health, and that schools remain safe with protocols such as required masks, weekly testing and improved air filtration. But the union argued that the district’s safety measures need to be stronger considering the surge. Among the points of contention are metrics that would trigger school closures.

While the union has characterized the vote as a return to remote instruction, district leaders called it a “walkout” and “illegal work stoppage.” A contentious battle took place last January over similar issues causing a bumpy start to the district’s return to in-person instruction after being remote since March 2020.

Martinez said buildings in the approximately 350,000-student district would remain open regardless of the union vote.

“If they do take a vote to do a walkout tomorrow, I have to cancel classes,” Martinez said at an afternoon news conference. “I’m not closing the schools, the schools will be open. All staff will be welcome to come to school because we are going to have a plan for our families.”

He later told reporters buildings would be open to administrators, staff and “essential services” but not instruction for students. He said more details would follow, like a plan for student nutrition services.

Both sides continued negotiations Tuesday with union votes, of both the House of Delegates and total membership, scheduled to end around 9 p.m. Families received robocalls, texts and emails Tuesday evening saying an update would come as soon as the outcome of the vote was known.

The district said in a statement that it has provided 200,000 KN95 masks to teachers, would allow schools to bring back daily health screening questions for students and building visitors that were required last academic year, and would spell out metrics for closing individual schools. For instance, the district said it would switch to remote learning at an elementary school if 50% of its classrooms had more than 50% of its students instructed to isolate or quarantine.

The Chicago Teachers Union, which has roughly 25,000 members, said it was reviewing the district’s offer, but that they received it “minutes” before the news conference. The union had sought the same metrics to close schools from an agreement last year. That includes a districtwide two-week pause on in-person learning if the citywide COVID-19 test positivity rate increases for seven consecutive days, for instance.

Union leaders said more safety protocols were needed and that the COVID-19 surge was causing staffing shortages. The district said roughly 82% of teachers reported to work Monday, which was lower than usual, but that classes were covered by substitute teachers and other staff.

“The layers of mitigation that we need to keep our schools open and keep our students inside of the school buildings have not happened here in Chicago,” the union’s vice president, Stacy Davis Gates, told CNN.

Roughly 100,000 students in the district are vaccinated and about 91% of teachers, according to the district.

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