SEATTLE — The finding was hard to believe, but year after year and in state after state, the numbers kept bearing it out: Sixth-graders who missed 20 days of class had, at best, a 20 percent chance of graduating from high school on time.

This was a bombshell for researcher Bob Balfanz, who’d spent most of his career trying to understand the factors driving 1 million American students to drop out each year. He’d paced school hallways and sat through hundreds of hours of classroom instruction.

But in 2007, after tracking 13,000 middle-schoolers for eight years in Philadelphia, Balfanz finally isolated a red flag common to all who, years later, failed to graduate on time: a history of poor attendance.

“You’d think, ‘Hey it’s only sixth grade, you can recover and grow out of this,’ ” he said.

Yet Balfanz, based at Johns Hopkins University in Baltimore, found that missing as few as 10 days a year has a cumulative impact, weakening the foundation upon which all other school achievement builds.

More surprising still, school attendance averages, while widely reported, are highly misleading. A district may accurately note rates of 90 percent, but still have hundreds of students missing weeks of instruction because that number is an aggregate – concealing the fact that different students are absent on different days.

“That’s when the sheer magnitude hit us,” said Balfanz, who recently released the findings of a major study on this effect.

Other researchers have found a similar relationship between poor attendance and low performance among rural kids, preschoolers and middle-class youth alike.

“Wherever we’ve looked, we’ve seen a clear relationship between missing a month of school and negative educational outcomes later,” Balfanz said. “That has been proven for all kids.”

Conversely, it turns out that targeting attendance can make a significant dent in such thorny areas as the achievement gap and high school dropout rates – without having to overhaul an entire curriculum.

This finding inspired Balfanz to create the anti-dropout program Diplomas Now, which sends dozens of recent college graduates into middle schools with a laser focus on attendance and tutoring. At Denny International and Aki Kurose in Seattle, both of which have long struggled with student discipline and lackluster scores, it is making a dramatic difference.

Until recently, their attendance figures were miserable.

In 2010, more than half of the 590 students at Aki Kurose missed at least 10 days of class. That may not sound like a crisis. But it meant two full weeks of instruction, and in math, where comprehension of basic principles is essential to building more-advanced skills, the fallout was predictable: 62 percent of students tested below grade level.

According to Balfanz, about 200 of them were high-school dropouts-in-the-making.

“The unexcused absence rate was ridiculous,” agreed Principal Mia Williams, who arrived in 2008 and quickly grasped the severity of the problem.

“The first thing we did was start paying attention to it, saying, ‘Hey, attendance really does matter. I mean, if you’re not here, how are you going to pass your classes?’ ”

In 2010, Williams and Jeff Clark, the principal at Denny, flew to Chicago to see Diplomas Now in action.

Four months later, they’d brought it home to Seattle Public Schools, and by the following school year, Aki Kurose achieved national attention for improving student attendance by more than 4 percentage points. Its trajectory has continued since.

The program – which targets course work and behavior, with truancy – has led to an 18-point rise in eighth-grade math scores in three years at Denny.