Tyrone Pittman, the dean of students at Simon Elementary School in Washington, has helped lead the school’s broad effort to reduce absences. (Evelyn Hockstein/For The Washington Post)

The nation’s large and persistent education achievement gaps are rooted in a largely hidden crisis of chronic absenteeism from school, especially among low-income and minority children, according to a new report that compiles recent research on school attendance.

School districts tend to focus on truancy, or skipping class. But that focus misses a big part of the problem, according to the report by two nonprofits: Attendance Works, a group that seeks to highlight the connection between attendance and academic success, and the Healthy Schools Campaign.

Absenteeism rates among kindergartners are nearly as high as those among high school freshmen, according to the report. An estimated 1 in 10 kindergartners misses at least 18 days of school, or nearly a month of class, per year.

Many of those absences are excused: Young children often miss school not because they’re skipping class but because they or their parents are suffering from mental or physical health problems.

Fourteen million absences — or one-third of all missed school days — are due to asthma, the leading cause of absenteeism, according to the Centers for Disease Control and Prevention. Tooth decay, which is five times more common than asthma, accounts for 2 million missed school days each year.

School districts and states that focus just on unexcused absences might not notice or intervene when a 5-year-old misses class frequently because of ill health. But chronic absence is harmful no matter its cause, the report says.

Kindergartners who are often absent miss critical instruction, especially in reading, making it hard for them to keep up with their peers. That contributes to a cascade of events that lead to a higher risk of dropping out: Attendance, even as early as first grade, can be predictive of graduation rates, according to a 2012 study.

“Poor attendance is among our first and best warning signs that a student has missed the on-ramp to school success and is headed off track for graduation,” the report says. “We must address attendance and its connection to public health early in a child’s life.”

Rhode Island officials tracked a cohort of kindergartners for seven years to discern the effects of chronic absences years later. They discovered that chronically absent 5-year-olds went on to lag their peers in later grades, scoring an average of 20 percentage points lower on reading tests and 25 points lower in math. They also were twice as likely to be retained a grade.

National data on chronic absenteeism among very young children is hard to come by. But the National Assessment of Educational Progress (NAEP), a nationwide exam for fourth- and eighth-graders, offers a glimpse of troubling patterns.

Students who take the NAEP are asked to report how often they were absent in the month before they took the exam. Nationally, black and Latino students are more likely than white students to report being absent three or more times in that month, and poor children are more likely to be absent frequently than affluent children.

Attendance has improved significantly in some communities that have made concerted efforts to understand and address chronic absenteeism with tactics including in-school health clinics and social workers, according to the report.

In New Britain, Conn., 30 percent of kindergartners were chronically absent, prompting the school district to hire two outreach workers focused on attendance. At the same time, principals began reviewing attendance data every 10 days in order to flag students in need of extra support.

Kindergarten absenteeism fell by half, according to the report, and early literacy improved.