AS THEIR COUNTERPARTS in Los Angeles, Washington and the rest of the nation cracked open their schoolbooks, students in Chicago spent the past five days locked out of their classrooms. The 350,000 students, most of them minority and poor, are the very children who can least afford to lose a week of instruction.

That the loss of this precious school time didn’t seem to faze the Chicago Teachers Union that fomented this reckless strike should come as no surprise: After all, the underlying message from its leaders has been that these children can’t be expected to learn anyway.

Negotiators for the Chicago public school system and the union emerged from talks Friday saying that the outlines of an agreement had been reached to end the strike by some 26,000 teachers and support staff. The two sides will meet again Saturday; both were optimistic that schools would open Monday. That would be welcome news to parents who lost work to stay home and children who had nowhere to go but trouble-plagued streets.

The details of the contract have yet to be released, but reportedly the major tenets of Mayor Rahm Emanuel’s reform effort are intact. Expect, as is the custom in labor disputes, for both sides to claim victory. It will be important to examine the fine print of issues such as how teachers are evaluated or assigned or paid, but even more critical is the mind-set that underlies the contract. What was most troubling about union officials resisting the kind of change promoted by President Obama and his Race to the Top was their rationale: their doubts — which essentially amount to a disheartening surrender — about the ability of students who come from poverty and face other difficulties to perform in school.

According to Karen Lewis, head of the Chicago Teachers Union, there are factors beyond the control of teachers — poverty, exposure to violence, homelessness, hunger — that prevent children from learning. “I wonder how well you would learn your ABCs in an overcrowded classroom where 10 percent of the children have asthma, 20 percent didn’t get a good night’s sleep and another 30 percent are recovering from witnessing a shooting in their neighborhood? Not to mention the number of students who haven’t had two solid meals since their last school lesson,” she wrote in the Chicago Tribune.

There’s no questioning the challenges these children face. There’s no disputing that they need help beyond school — nutritional, parent counseling and more. But give such children strong teachers with high expectations in a well-led school and, as evidence and experience have shown, they can and do succeed. What matters is what educators do in the face of poverty. That’s why Mr. Emanuel was right to push for an evaluation system that differentiates among teachers and values those who are effective in getting children to learn.

No one thinks that teachers’ whole careers should be based on one standardized test. Evaluations must be fair, tests need to be improved and standards must be tightened. But unions need to stop blaming poverty and become part of the solution. If they don’t think they can, they should stop standing in the way of charter schools that give parents the option of teachers who believe their children can learn.