Chicago's teachers union voted on Tuesday to return to remote learning amid a new surge in coronavirus cases, forcing classes to be canceled on Wednesday, district officials said. 

The vote was approved by 73% of the union's members, calling for no in-class learning until "cases substantially subside" or union leaders approve an agreement for safety protocols with the district. 

The Chicago school district is the nation's third-largest. Students in the district had just returned to classes on Monday after a two-week winter break. 

"This decision was made with a heavy heart and a singular focus on student and community safety," the union said in a statement late Tuesday.

Chicago Public Schools officials had insisted on keeping schools open for in-person classes, saying remote instruction during the pandemic has been disastrous for children’s learning and mental health. 

The union argued schools didn't have proper safety protocols in place amid the surge. 

COVID-19 OMICRON VARIANT STEPS ON PLANS TO RETURN TO SCHOOL 

Charles H. Wacker Elementary School in Washington Heights, a Chicago community on the Far Southwest Side

Charles H. Wacker Elementary School in Washington Heights, a Chicago community on the Far Southwest Side (iStock)

COVID-19 cases have surged across the country in recent weeks after the discovery of the highly contagious omicron variant. 

Chicago Teachers Union workers direct cars lined up for COVID-19 testing outside CTU headquarters on Thursday, Dec. 30, 2021

Chicago Teachers Union workers direct cars lined up for COVID-19 testing outside CTU headquarters on Thursday, Dec. 30, 2021. The union held its own testing clinic amid concerns about inadequate COVID-19 protections as classes resume following winter break.  (Stacey Wescott/Chicago Tribune/Tribune News Service via Getty Images)

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 it first went remote in March 2020.

Schools CEO Pedro Martinez said buildings would remain open regardless of the union vote, saying buildings were open for administrators, staff and "essential services" but not instruction for students. Mayor Lori Lightfoot also signaled that teachers who did not show up to work would be placed on "no pay status."

The school district said it has provided teachers 200,000 KN95 masks and would allow a return of daily COVID-19 testing in response to the concerns of the union's 25,000 members, but the union said it received the district's offer soon after the vote and would have to review it first. 

Lightfoot urged teachers to show up for classes.

"I want to assure you that I am doing everything in my power to keep our students in school, where they belong, learning," Lightfoot, a Democrat, said to parents on Twitter Tuesday. 

She said her administration is working with the union to negotiate a fair agreement. 

"But what we cannot accept is unilateral action to shut down the entire district, depriving hundreds of thousands of students of the safe, in-person schooling environment they need."

Her administration decided to cancel classes completely rather than revert to virtual learning, according to The New York Times.

"Nobody signs up for being a home-schooler at the last minute," she said Tuesday. "We can’t forget about how disruptive that remote process is to individual parents who have to work, who can’t afford the luxury of staying home."

She said in a press conference Tuesday evening that bouncing between in-person and remote learning isn't "fair" to parents and it "destabilizes the system." 

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Union leaders said more safety protocols were needed and that the COVID-19 surge was causing staffing shortages. The district said roughly 82% of its roughly 21,600 teachers reported to work Monday, which was lower than usual, but that classes were covered by substitute teachers and other staff.

The Associated Press contributed to this report.