The Chicago teachers’ strike was prompted in part by a fierce disagreement over how much student test scores will weigh in a new teacher evaluation system mandated by state law. That teachers’ unions in much of the country now agree that student achievement should count in evaluations at all reflects a major change from the past, when it was often argued that teaching was an “art” that could not be rigorously evaluated or, even more outrageously, that teachers should not be held accountable for student progress.

Traditional teacher evaluations often consist of cursory classroom visits by principals who declare nearly every teacher good, or at least competent, even in failing schools where few if any children meet basic educational standards.

As a result of this system, bad things can happen. High-performing teachers who have an enormous impact on student achievement go unidentified, and they often leave the district. Promising, but struggling, young teachers never get the help they need to master the job. And disastrous teachers who have no feel for the profession continue as long as they wish, hurting young lives along the way.

The more rigorous evaluation systems that have taken root in several states and districts around the country are intended to change that picture. These systems, which take student achievement into account in various ways, are still in their formative years, but they have already opened the door to a different way of doing business. At their best, these evaluation systems are based on the idea that teaching is difficult to master and that high-performers tend to get that way through intensive feedback and help from colleagues.