CHICAGO (AP) — Chicago Public Schools plans to proceed with the reopening of elementary and middle schools on Monday despite the failure to reach an agreement with the teachers union, Mayor Lori Lightfoot said Friday night.

School officials and the Chicago Teachers Union have been locked in negotiations for days in an attempt to reach an agreement to reopen schools closed in March to about 335,000 students because of the COVID-19 pandemic. The teachers union has opposed the school district’s plan over fears for the health of its members.

Lightfoot said late Friday that the two sides have agreed on several issues. But union leaders are balking at putting certain points of agreement in writing so that the two sides can work on other issues they disagree on, she said.

In-person classes were canceled this week for about 3,200 pre-K and special education students when teachers refused to work in classrooms. Officials say they expect those students to return to class on Monday.

“Those teachers need to be there,” for those students, Lightfoot said. If not, “we will take further action,” though she declined to elaborate.

School officials argue remote learning isn’t working for all students, including many low-income and Black and Latino students who make up the majority of the district. The district’s safety plan includes thousands of air purifiers, more cleaning and a voluntary testing program.

Union officials, though, say returning to in-person instruction before its members are vaccinated and without other safeguards in place would put them at greater risk of contracting the virus. They argue that if the district tries to punish teachers for staying home Monday, then the district would be responsible for a work stoppage.

The union also rejected Lightfoot’s contentions, saying in a tweet that the two sides were working toward an agreement around key components.

“In the last hour, the mayor has wrecked it all,” the union said in its tweet.