While foster youth miss more school versus their non-foster counterparts, their status as a foster youth is not static, with many of them entering and exiting the foster care system over time. These dynamics of entry and exit can represent particularly crucial transition periods of stability and instability that may differentially influence absenteeism. Yet, there is a dearth of studies that have explored these dynamics; absent this knowledge, we may be overlooking children in the child welfare system that may need targeted support during these transition periods to promote their educational wellbeing. In our study, we estimate whether and how entry into and exit out of foster care have different associations with absenteeism. We analyze four years of longitudinal data (2015-16 to the 2018-19 school years) from four large districts in California, the state with the largest population of foster youth, using an asymmetric fixed effects strategy. For children involved with the child welfare system, entering foster care is associated with 2.5 fewer days absent (-2.51; p < 0.001) while the probability of being chronically absent was lower by about 12.6 percentage points (-0.126; p < 0.001). On the other hand, foster youth exiting foster care missed about an additional day (0.958; p < 0.01) while their probability of chronic absenteeism was about 6 points higher (0.058; p < 0.001). Overall, these results shed new light about the vulnerability of foster youth exiting the system and bring new awareness of targeted supports they may need to promote their attendance.