SOME 26,000 TEACHERS and support staff in the nation’s third-largest school district went on strike Monday after months of negotiations failed to produce an agreement. It is Chicago’s first teacher strike in 25 years. Caught in the middle are about 400,000 students, whose families had to scramble to keep them safe and occupied. Negotiations resumed Monday, but prospects for a settlement are unclear.

The strike, which Mayor Rahm Emanuel labeled one of “choice” by labor leaders, caught many by surprise because teachers, who on average make about $75,000, had been offered a 16 percent average salary increase over the next four years, and because the city had made a series of concessions. But the union dug in its heels at accepting changes in teacher evaluation procedures or in allowing principals the right to select school staff.

Mr. Emanuel, who previously battled with union leaders over his bid to lengthen Chicago’s notoriously short school day, is right that hiring decisions are best left to the people who will be held accountable for the results. Who, other than union leaders, think it’s a good idea to force principals to hire from a pool of people whose only qualification is previous service and who in many cases were deemed to have been ineffective? No parents want their children taught by such a teacher. No qualified teacher wants to inherit that teacher’s students the next year.

The mayor is right also that teachers should be judged in part on their success in promoting achievement. Illinois law requires the use of student test scores in teacher evaluations. The system developed by Chicago officials, on which they offered to work with the union, is careful to measure student growth. That means teachers aren’t blamed if their students start out behind but instead are evaluated on their ability to make progress during the year they have responsibility.

Events in Chicago are being closely watched. The city is home to both President Obama and his education secretary, Arne Duncan; Mr. Emanuel is the president’s former chief of staff, and the teachers union plays a prominent role in presidential politics. The administration has championed reforms much like those the Chicago local is fighting. And with good reason: A scandalously low 56 percent of Chicago students graduate from high school. That is the status quo the union is fighting to preserve.