For schools and teachers to help students develop knowledge and skills, students need to show up to class. Yet absenteeism is prevalent, especially in secondary schools. This study uses a rich data set tracking class attendance by day for over 50,000 middle and high school students from an urban district in academic years 2007-2008 through 2012-2013. Our results extend and modify the extant findings on absenteeism that have been based almost exclusively on full-day absenteeism, missing class-by-class absences. Notably, part-day absenteeism is responsible for as many classes missed as full-day absenteeism, raising chronic absenteeism from 9% to 24% of secondary-grades 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 whereas part-day absenteeism remains high in Grades 10 and 11 and increases again in Grade 12. Whereas 55% of full-day absences are unexcused, 92% of part-day absences are unexcused. Absenteeism from individual classes varies considerably by time of day but less by class subject matter.