Mexican students studying to be teachers released a hostage on Wednesday—in the municipality of Nahuatzen—due to concerns about his health. But they continue to hold five others. The students are supported by the Michoacán State Teachers Organization, which warned that the remaining captives, who are state policemen, would be freed only when a demand for 1,200 new teaching jobs is met.

The Mexican standoff, now a week old, is only the latest example of a teacher-union rebellion against recent amendments to the Mexican constitution...