The Chicago Teachers Union has reached a tentative agreement with Mayor Lori Lightfoot to reopen the city’s schools for personal teaching, the mayor said on Sunday.

When completed, the deal would stave off a strike that threatened to disrupt classes for students in the country’s third largest school district.

As part of the deal, preschool kindergarten and some special education students would return to classrooms on Thursday. Kindergarten staff through fifth grade classrooms would return on February 22, and students in those classes would return on March 1. Staff in sixth through eighth grade classrooms would be returning on March 1 and students on March 8.

The deal must be approved by the union’s elected governing body, the House of Delegates, the mayor said. The union leadership is expected to meet with their base on Sunday afternoon, and then the House of Representatives will meet, according to a person with knowledge of the situation, who spoke on condition of anonymity because the union did not want the deal before the public Members had the opportunity to see it.

The Chicago Tribune reported the existence of the deal on Sunday morning. Shortly thereafter, the union posted on Twitter: “We don’t have an agreement with the Chicago Public Schools. The mayor and her team made an offer to our members yesterday evening that requires further review. We will continue our democratic process of simple scrutiny throughout the day before an agreement is reached. “

Mayor Lightfoot and the union were embroiled in one of the most intense reopening battles in the country. The mayor has argued that the city’s most vulnerable students needed the opportunity to return to school in person, while the union condemned the city’s reopening plan as unsafe.

A similar battle is underway in Philadelphia, where pre-school through second grade teachers are due to report to school buildings on Monday in preparation for the return of students on February 22nd. The teachers’ union there has advised its members to continue working remotely. It was not yet safe to return to school buildings.

Ms. Lightfoot said Sunday that the fight with the union in Chicago had been bitter. She said she heard from parents who felt they were being held hostage and drowned out their voices. She tried to bring the vitriol to the past.

“My Chicagoans, we have to move forward and we have to heal,” she said.