Chicago Public School students who want to graduate will have to show proof that they have a plan after high school—such as providing an offer letter for a job or acceptance into college or military service, under a plan expected to be approved next month.

The initiative, pitched by former U.S. Education Secretary Arne Duncan and carried by Chicago Mayor Rahm Emanuel, is targeted at encouraging students to plan for life after high school. It likely goes farther than any other public-school initiative to encourage postsecondary...