Robert Balfanz, the other co-author of the report and the director of the Everyone Graduates Center at Johns Hopkins University, said districts with high levels of chronic absenteeism are located in diverse settings, from large urban centers and affluent suburbs to small post-industrial towns and rural enclaves. “We have to know where we have to go to find these kids who are chronically absent, and tailor our approach to the situation,” he said. “A big benefit of this report is it helps us understand the contours of chronic absenteeism.”

In the Philadelphia school district, 37 percent of the system’s 144,000 are chronically absent. Among high-school students, the figure shoots up to 51 percent. The districts in Baltimore and Milwaukee have similar numbers. For Cleveland and Detroit, the chronic absenteeism rates are around 50 percent, and more than 60 percent of Cleveland’s high-schoolers missed more than three weeks of school a year.

The report’s authors write that one common denominator linking these cities is the “nearly 100 years of historical actions that aimed to segregate African American populations in sections of the city with the poorest housing, greatest proximity to industrial pollutants, greatest exposure to violence, and highest unemployment rates, resulting in widespread inter-generational poverty.”

Tyrone Howard, an associate dean of inclusion and an education professor at the University of California, Los Angeles, said in an email that “chronic absenteeism is typically one of the most ardent signs that something serious may be happening with a student and his or her family.” He noted that students who skip school may be caring for younger siblings, experiencing neglect or homelessness—a plight endured by 1.3 million U.S. students—or facing related traumas.

“Such episodes can lead to students avoiding school for long periods of time,” he added.

Some districts with high levels of chronic absence have more resources than others. Chang and Balfanz’s research shows that affluent suburban districts, such as those in Fairfax County, Virginia, and Montgomery County, Maryland, post some of the highest absenteeism numbers in the country, likely as a result of their growing low-income populations, though the communities’ wealth can provide the financial backstop to support students who are missing too much class. In fact, recent reporting from U.S. News & World Report found that some rich suburban districts receive more federal aid for low-income students than smaller but far poorer districts.

Also at risk are the hundreds of thousands of students in smaller, post-industrial cities, “places like Utica, Schenectady, and Albany in New York, and Syracuse and Buffalo, all of them have very high rates of chronic absenteeism, often topping 50 percent, and they all often have very high rates of concentrated poverty," said Balfanz. "That's part of the challenge, that many states have these sort of economically and socially isolated cities spread out across the state."