For the past generation, the federal government has rated schools by their standardized test scores. However, states now have more room to decide for themselves, and many of them are choosing to judge schools in part based on how many kids miss class.

More than 6 million children missed more than 15 days – or three weeks – of school during the 2013-14 school year, according to federal data published last year. This was the first time nationwide data like this has been gathered and published.

Johns Hopkins University researcher Robert Balfanz is hopeful that incorporating absenteeism in school rating systems will encourage districts and school leaders to pay more attention to this overlooked problem.

“If kids aren’t there every day, it’s going to be hard for us to succeed,” said Balfanz, whose work has helped show that there is a tight link between students who miss class frequently — more than 10 percent of the school year, or 18 days in a 180-day school year — and those who struggle academically and eventually drop out.

Many states are choosing to use absenteeism both because it is linked to student achievement and because it doesn’t require a lot of new resources, since schools and districts already collect attendance data, said Chris Minnich, executive director of the Council of Chief State School Officers.