Last year, nearly a quarter of all students in Cohoes missed at least 18 days of school. That's two days a month, or 10 percent of all days in a school year. It's also a telltale sign of chronic absenteeism, a hidden epidemic that studies show affects students far beyond their school career.Students who are chronically absent — meaning they miss at least 15 days of school a year — are not only at serious risk of falling behind in school, data show, but are more likely to drop out of high school altogether, experience poverty, suffer worse health and end up in the criminal justice system later in life.