After parents and teachers opposed a hybrid model, Chicago schools will reopen online only.

Public school students in Chicago, the nation’s third-largest district, will begin the academic year remotely in September, leaving New York City as the only major school system in the country that will try to offer in-person classes when schools start this fall.

Mayor Lori Lightfoot of Chicago and Dr. Janice Jackson, the chief executive of Chicago Public Schools, made the announcement Wednesday morning, as the Chicago Teachers Union was in the midst of tentative preparations for a strike over school safety.

The school district had originally planned to open using...