The district on Sunday evening ordered teachers of kindergarten through eighth grade to return to their school buildings on Monday and threatened to cut off their access to remote-learning software if they didn’t. The union said late Sunday that it would consider any such move an illegal lockout and its leadership would consider next steps, which it has said in the past could include a strike.

The nation’s third largest school system had planned to relaunch in-person learning for kindergarten through eighth-grade students on Monday, but delayed the call for students to return to Tuesday. Teachers, though, were told they needed to return to classrooms to prepare, despite heavy snowfall Sunday that made travel difficult.