If showing up is half the battle, a lot of American schoolchildren are in trouble.

A new analysis of data collected by the Education Department’s Office of Civil Rights from the 2013-14 school year shows that more than 6.5 million students, or about 13 percent in grades K-12, missed 15 or more days of school.

The study revealed that instances of chronic absenteeism are found in almost every school district, but that half of the nation’s chronically absent students were found in just 4 percent of districts.

The report, by Attendance Works, a national educational initiative that looks at the relationship between attendance and outcomes, and the Everyone Graduates Center at Johns Hopkins University School of Education, also found that the problem of chronic absenteeism disproportionately affected African American, Native American, Pacific Islander and Latino students, as well as students with learning disabilities.

The scope of the absenteeism matters “because chronic absence is really a proven early indicator of academic risk starting as early as preschool and kindergarten,” Hedy Chang, executive director of Attendance Works, said in a conference call. “By middle and high school, it is a surefire predictor of kids being on the path to drop out. And if it reaches high levels, the classroom churn can affect the learning of all the students in the classroom, not just those who are chronically absent.”

The study set out to map where the chronic absentee students are located, how they are concentrated and the characteristics of districts with high numbers or concentrations of students who regularly miss school.

Among the most harshly affected areas were poor, urban school districts such as Philadelphia, Baltimore, Milwaukee, Cleveland and Detroit, where more than a third of all students were chronically absent. Smaller, post-industrial cities such as Buffalo and Utica, N.Y., were also hard hit, with absentee rates often topping 50 percent.

“These are all places that had concentrations of intergenerational poverty and a web of systematic challenges that just make it more difficult to come to school: poor housing, poor access to health [care], too much exposure to violence, greater exposure to environmental pollutants,” said Robert Balfanz, director of the Everyone Graduates Center.

A surprising discovery for researchers was the number of suburban districts considered mostly affluent and successful that were also hard hit by absenteeism, including Montgomery County, Md., and Fairfax County, Va.

Both were among the top 15 school districts in the country in total number of chronically absent students. They are much larger than many other school districts and have a growing number of low-income residents.

Poor, rural districts with higher white populations also had high rates of chronic absenteeism, though because the districts were smaller, fewer students were affected.

Until recently, chronic absenteeism was overlooked as a factor in performance and student outcomes, said Chang, adding that she hopes it is now recognized as a deciding element.

“All the best instruction in classrooms just doesn’t make a difference in classrooms if kids aren’t there to benefit from it,” she said. “Most people do not realize that just missing two days a month can throw you off track for academic success.”

For the researchers, understanding the absentee data is essential in helping form solutions appropriate to each district.

“Every district, every state has to know the size and concentration of their chronic absence problem and know how severe it is,” Chang said, “because that information will help them think about targeting of resources. If you know where your chronic absenteeism is happening, it can allow you to think about how you want to bring in other community partners to solve the problem.”