School attendance is essential to academic success, but too often students, parents and schools don’t realize how quickly absences, excused or unexcused, can add up to academic trouble. Chronic absence— missing just 18 days per school year— can leave third graders unable to master reading, sixth graders failing courses and ninth graders dropping out of high school. The impact is the greatest on low-income students who lack the resources to make up for the lost time in the classroom.