We examined the association of students' ability to transition on a daily basis (horizontal transitions) as reported by teachers with academic achievement as mediated by the summer Extended School Year (ESY) services received. Transitions from summer break to re-entry in the fall are referred to as a vertical transition. We examined these relationships via structural equation modeling techniques using multi-group modeling with respect to students with Individualized Education Programs (IEPs) versus students without IEPs. Among students with IEPs, increased ability to transition was significantly associated with increased academic achievement across time. The receipt of summer ESY services was strongly and significantly associated with increased academic achievement among students with IEPs. The receipt of summer ESY services was also significantly associated with increased ability to transition for these students with IEPs. This ability to transition may be even more impacted by the advent of the COVID-19 pandemic, which has disrupted learning for many school children.