For schools and teachers to help students develop knowledge and skills, students need to show up to class. Yet absenteeism is high, especially in high schools. This study uses a rich dataset tracking class attendance by day for over 50,000 middle and high school students from an urban district in Academic Years 2007-'08 through 2012-'13. Our results extend and modify the extant findings on absenteeism that have been based almost exclusively on data on full-day absenteeism, missing class-by-class absences. Notably, part-day absenteeism is responsible for as much class time missed as full-day absenteeism, raising chronic absenteeism from 9 to 24 percent of total students. Incorporating part-day absences sharply increases the chronic absenteeism gap between underrepresented minority students and their peers. Both full- and part-day absenteeism show a discrete jump at the point of transition from middle school to high school, but full-day absenteeism then declines while part-day absenteeism remains high in grades 10 and 11 and increases again in grade 12. While 55 percent of full-day absences are unexcused, 92 percent of part-day absences are unexcused. Absenteeism from individual classes varies considerably by time of day, but less by class subject matter.