A few weeks after the release of the Mackinac Center's high school report card, the Michigan Department of Education released its revised school ratings, with a new focus on the academic achievement gap among high-performing and low-performing students.

Schools that have the largest gaps between the top-scoring 30 percent of students and the bottom-scoring 30 percent of students are now labeled "focus schools," and face increased scrutiny from the state. This new measure has identified 358 such schools.

It certainly is important for students who come from an economic disadvantage to receive a quality education and to have the opportunity to grow as much as possible. But a narrow focus on the achievement gap can mask mediocrity and discourage diversity.

Looking at a snapshot of the achievement gap among students will ignore student growth. If young students enter the school with a wide range of academic abilities, a snapshot of that gap will tell you nothing about whether the school does a good job of educating those students, or reducing that gap over time.

Moreover, the achievement gap focus may penalize districts participating in Michigan's "schools of choice" program. If a district participates in schools of choice, students who do not live within that district can choose to attend it, and state funding will follow those students to their preferred schools. Students from disadvantaged backgrounds may be more likely to choose a better school, and this behavior could result in increased academic achievement gaps in high-performing districts that have a generous choice policy.

The achievement measure will also overlook broad failure. Schools that have higher and wide ranging student achievement across all student groups may be identified as focus schools, while schools where all students are doing poorly may be ignored.

For example, 27 of the high-performing Ann Arbor Public Schools' 32 schools were identified as focus schools, while no schools from the Highland Park School District or Muskegon Heights Public Schools were identified as focus schools. Is it acceptable for just 25 percent of seventh grade students to score proficient on state standardized tests if there is no achievement gap among them? 

Michigan is not alone in having an enthusiastic focus on the student achievement gap. Rick Hess, director of education policy studies at the American Enterprise Institute, wrote in 2011 that, in a "terrible irony," the national focus on the achievement gap can make it more difficult to promote integrated schools and can drive schools to disregard subjects that are not a component of  achievement gap measures.

What Hess dubbed "achievement-gap mania" can have the sad and unintended consequence of deteriorating school quality. Instead, Michigan schools should focus on what is important: How much each individual student learns over the course of a year, and figuring out how to help students learn as much as possible.

On Tuesday, at a conference to discuss reworking the way the state of Michigan directs funding to schools, State Superintendent of Public Instruction Mike Flanagan said, "We're not interested at all in raw test scores; we're interested in growth."

If his words are any indication, Flanagan is on the right track: Focusing on student growth instead of gaps could help drive schools to serve all students. Indeed, overall student academic growth in Michigan appears to be lower than it was just a couple years ago. That is the real problem that must be addressed.