Education Secretary Arne Duncan answers questions after speaking about the administration's priorities for education at Seaton Elementary in Washington, Monday, Jan. 12, 2015. (Jacquelyn Martin/AP)

EDUCATION SECRETARY Arne Duncan tells a story that was distressingly familiar for children of color before the implementation of the No Child Left Behind law. At his mother’s after-school tutoring program on the South Side of Chicago, he worked with a young man who thought he was headed to college. The student was on track to get his high school diploma — on the honor roll, in fact, with a solid B average. But, Mr. Duncan discovered, his tutee also was functionally illiterate.

In those years, no one was held accountable for student achievement, and schools routinely ignored and concealed the problems of struggling students, especially poor black and Hispanic students. Returning to that way of operating should be unthinkable, but that is unquestionably what will happen if testing and accountability requirements are gutted from federal law.

Rewriting No Child Left Behind is a priority for Congress and the administration. Lamar Alexander (R-Tenn.), chairman of the Senate education panel, aims to bring a bill to the Senate floor next month. Everyone agrees that the law, a product of bipartisan cooperation between President George W. Bush and the late Sen. Edward M. Kennedy, should be improved based on the experience of the past decade-plus. Some of its provisions are overly punitive or prescriptive. But its underlying principle is sound: Schools need to know whether students are learning, and they should do something about it when they aren’t. Writing that principle into law was a civil rights achievement that must not be undone.

The achievement is threatened by an unholy alliance between conservatives who oppose any federal role in education and teacher unions, which claim to favor progressive policies until those policies begin to bring accountability to the teaching profession. These opponents of the law, blowing a lot of smoke about “corporatist” reform, would like to eliminate the requirement that students be tested annually in math and reading in grades three to eight and once in high school. Mr. Alexander’s draft bill doesn’t take a stand on this, instead presenting a range of options in what the senator characterized as an attempt to start a dialogue about testing.

That dialogue should start with a fact: The law has worked. The performance of poor and minority students has improved in the past 10 to 15 years. The Education Trust, advocates for closing the achievement gap, has catalogued the evidence in the performance of minority and low-income students on the National Assessment of Educational Progress, commonly described as the nation’s report card; in a rise in graduation rates; and in greater participation in college entrance and Advanced Placement tests.

Some districts may test too often or teach too much to the test. There is room to fix problems and, as we said, improve the law. But any member of Congress should be embarrassed to even contemplate returning to an era when the absence of annual measurement allowed failure to be swept under the rug. Educational opportunity is, as Mr. Duncan said Monday, “a civil right, a moral imperative.” The country needs to ensure that no one is being denied that right.