ANYTIME THERE is a suggestion a school may have cheated its way to showing improved student achievement, there is reason for serious concern. That’s why D.C. school officials hired a high-priced outside expert to investigate what appeared to be anomalies on a number of student test sheets. It’s also why it is prudent for the system to take another look at the schools where tests were called into question. But to use the issue of erasure marks at a handful of schools to disparage the very real improvements made in recent years by D.C. schools is irresponsible.

Acting Schools Chancellor Kaya Henderson on Tuesday asked the city’s inspector general to determine whether there were any improprieties at eight schools on standardized reading and math exams in 2009. Her request follows disclosures by USA Today of suspicious rates of erasures in which wrong answers were changed to right ones. The report centered on Crosby S. Noyes Education Center in Northeast Washington, credited with dramatic boosts in student achievement. There were extraordinarily high numbers of erasures for three years at the school. One Noyes classroom averaged 12.7 wrong-to-right erasures per student on a 2009 reading test when the average for seventh-graders in all D.C. schools on that test was less than one.

Experts caution against any simplistic reading of erasure rates; there are many innocent explanations for changed answers. Indeed, it was the finding of Caveon Consulting Services, the testing security firm hired by D.C. schools, that there were plausible explanations and no evidence of misbehavior at Noyes and the other schools investigated.

The city hired Caveon, at an annual cost of $100,000, after it was embarrassed by its inability to investigate 2008 test anomalies; the company’s charge was to improve testing security and investigate any cases flagged by the firm that scores the test and referred by the Office of the State Superintendent of Education. John Fremer, president of Caveon, described the investigations — which included interviews with school personnel and data analysis — as “a thorough job working with credible data.” He told us that if there were reason to believe something was amiss, the company would have advised further action. Further scrutiny will show whether the city got its money’s worth on this investigation.

That the schools were cleared by an outside firm seen as premier in its field, or that they represent just 2 percent of the system’s total testing, hasn’t stopped critics of the reforms begun by then-Schools Chancellor Michelle A. Rhee from seizing on the situation as evidence that the improvement in D.C. schools is a myth. Other tests — including the National Assessment of Educational Progress, about which there are no questions — showed significant gains in reading and math by D.C. students between 2007 and 2009. That’s evidence of progress that can’t be erased.