There are many worrisome things in a new report by university researchers who studied the A-F school grading system in Oklahoma. Among other things, they found that very small variations in test scores could mean the difference between an A grade and an F grade for a school.

These grading systems are central to the No Child Left Behind Act waivers that most states, including Oklahoma, have been granted by the U.S. Department of Education. These new ratings, whether they be letter grades or stars or rankings, take the place of the “adequate yearly progress” or AYP that schools were supposed to make sure all of their students, including subgroups of at-risk students and minorities, made toward grade-level standards. And so in the Oklahoma study, this conclusion seems most worrisome:

“The study says that when examining students receiving federally subsidized meals, they actually scored higher in D and F schools on average than they did in A and B schools, meaning that these supposedly top schools are ‘masking the especially low performance of poor and minority children’,” my colleague Andrew Ujifusa writes.

This is what civil rights and other education advocacy groups have feared about the new waivers, which have allowed states to create “super subgroups.” These super subgroups often comprise the lowest 25 percent of students in every school, or all racial minorities clumped together. There were two benefits to this approach: First, it allowed schools to get around some of the statistical problems (and resulting loopholes) that small subgroups caused under the existing NCLB, and second, it prevented certain students—such as Latino student learning English and qualifying for subsidized meals—from counting multiple times in the accountability determinations.

But advocacy groups have worried that turning away from these smaller subgroups would allow some schools to hide behind the performance of their top students. This might be the first study, under the new waivers, to provide some evidence that there’s real reason to worry.

To be sure, the Education Department is aware of these concerns and plans to closely scrutinize the practical effects of these grading systems, both in identifying the correct schools for interventions and in making sure poor subgroup performance is not overlooked.

But the early research in Oklahoma shows just how much more study is needed of these grading systems, which are now much more prevalent than the AYP system.