While an emerging body of research has examined the effects of school absences on student outcomes, there is comparatively little research examining the different reasons contributing to school absence, how common these reasons are, and the extent to which different types of absences are differentially associated with achievement. To address these gaps, we used data from the Longitudinal Study of Australian Children to examine the reasons for school absence as reported by 14-15 year olds and how these reasons relate to achievement outcomes in Year 9. Only 7% of 14-15 year olds indicated they had been absent in the previous six months without parental consent, of which 46% indicated the most recent absence was due to problems at school. Of the 90% of students who had been absent with parental consent, only 6% said the most recent absence was due to problems at school. After controlling for student, family and school characteristics and Year 7 achievement, Year 9 achievement was most strongly associated with absences related to student- or family-level reasons. While schools typically bear the responsibility for monitoring and responding to absenteeism, the drivers of absence may not be related to factors that schools can realistically address. For schools, addressing absenteeism requires a dual approach of preventing avoidable absences and mitigation strategies for when either avoidable or unavoidable absences occur.