Up to 7.5 million students miss nearly a month of school each year, making them much more likely to drop out before they graduate, according to a new national report.

What's more startling is that only six states track chronic absenteeism in schools, according to a study by Johns Hopkins University researchers released Thursday at events in Philadelphia and Washington, D.C.

"No one is measuring this most fundamental thing _ are kids attending school regularly" said Robert Balfanz, one of the Johns Hopkins researchers who worked on the study. "You can't even analyze what's working in closing the achievement gap without looking first at chronic absenteeism."

The study marks the first time researchers have tracked the prevalence of students who habitually miss school.

The researchers estimate that up to 15 percent of students nationally are considered chronically absent. That spikes up to one-third of students in urban and rural areas, where students may be poor and come from families with little education.

As a result, students who typically need more help to be successful academically are the ones missing the most school.

Researchers found absenteeism to be highest in kindergarten and in high school.

A student is considered chronically absent if they missed at least a month of school in a year, or about 10 percent. Some schools had the equivalent of entire classrooms of students missing that much or more each year, the researchers found.

The federal government doesn't track the problem, and neither do most states. The only states that keep up with it are Georgia, Florida, Maryland, Nebraska, Oregon and Rhode Island.

Because of the limited data, Balfanz cautions that the study is an "educated guess" about the scope of the problem.

The U.S. Department of Education requires states to track daily attendance, but those numbers don't reflect students who are chronically absent.

For example, a school can have a 90 percent attendance rate but still have hundreds of students who are chronically absent because the students all miss different days.

"The research shows that we must address the attendance problem if we are going to have the kind of broader school improvement we want and our students deserve," said Marie Groark, executive director of Get Schooled, an education nonprofit that paid for the study.