from their middle school students. As schools ramp up their academic focus, however, the analysis shows the cost of missing school may be greater.

Fifty-six percent of 8th graders who performed at the advanced level in NAEP reading in 2011 had perfect attendance in the month before the test, compared with only 39 percent of students who performed below the basic level.

In comparison, nearly one in five 8th graders at the basic level and more than one in four below basic in reading had missed three or more days in the past month, according to Alan L. Ginsburg, a research consultant for the governing board and a co-author of the report with Naomi Chudowsky of Caldera Research in Bend, Ore.

"Three days, if you multiply that out by nine months,the principal and chief executive officer of the Dr. Martin Luther King Jr. Charter School for Science and Technology, in New Orleans.

Low Performers

Academic expectations seem to be increasing for middle school students both in school and at home, the researchers found. But the bulk of the additional instructional time happened before the 2001 passage of the federal No Child Left Behind Act, with its new demands for academic progress, and the students who most needed extra time weren't always the ones to get it.

Teachers reported that from 1996 to 2000, 18 percent of 8th graders moved from having less than four hours of mathematics instruction each week to four or more hours a week, and from 2005 to 2011, another 6 percent of students started receiving five or