Few American cities have labor politics as fraught as Chicago’s, where the nation’s third-largest school system shut down this week after the teachers’ union members refused to work in person, arguing that classrooms were unsafe amid the Omicron surge.

“I think that Joe Biden and Miguel Cardona and the newly elected mayor of New York City and Lori Lightfoot — they can all declare that schools will be open,” Ms. Mizialko added, referring to the U.S. education secretary and the mayor of Chicago. “But unless they have hundreds of thousands of people to step in for educators who are sick in this uncontrolled surge, they won’t be.