Teachers’ strikes, because they hurt children and their families, are never a good idea. The strike that has roiled the civic climate in Chicago — and left 350,000 children without classes — seems particularly senseless because it is partly a product of a personality clash between the blunt mayor, Rahm Emanuel, and the tough Chicago Teachers Union president, Karen Lewis. Beyond that, the strike is based on union discontent with sensible policy changes — including the teacher evaluation system required by Illinois law — that are increasingly popular across the country and are unlikely to be rolled back, no matter how long the union stays out.

Mr. Emanuel attracted the union’s anger when the city, citing budget deficits, rescinded a 4 percent raise that was supposed to go into effect last year. He further angered the union by bypassing the collective-bargaining process with a new policy that lengthened one of the shortest school days in the nation. Comparatively speaking, however, Chicago’s teachers are well paid, with an average salary of about $75,000 a year (roughly the same as in New York City). Before the strike, the city agreed to increase the size of the teacher corps to handle the longer school day. And despite its dismal fiscal condition, the city says it has offered the union a 16 percent raise over the next four years.

The union has listed several grievances in its public statements, but the main point of anger has to do with a state law that requires school systems to put in place an evaluation system in which a teacher’s total rating depends partly on student test scores. Half the states have agreed to create similar teacher evaluation systems that take student achievement into account in exchange for grants under the federal Race to the Top program or for greater flexibility under the No Child Left Behind law. Such systems are already up and running in many places.

In Chicago, however, the union asserts that the city’s evaluation system will unfairly penalize teachers for fluctuations in student performance that might be attributable to family crises or even neighborhood violence. For its part, the city says it is willing to monitor the new system to ensure fairness and negotiate a grievance process that would allow the teachers to challenge their ratings. If the union has legitimate suggestions for how to improve the fairness and accuracy of the evaluation system, in particular, it needs to bring them forward in a way that does not involve holding the city hostage.