The phenomenon of summer slide or setback has gained a great deal of attention in the USA. It is understood to account for as much as 80% of the difference in achievement for students between low and high socio-economic families over their elementary schooling. In a mixed method longitudinal study of reforms in low socio-economic school communities in Victoria, Australia this phenomenon in the achievement growth of primary and secondary school students for both literacy and numeracy was identified. The longitudinal analysis of achievement data revealed decelerated growth during Terms 4 and 1, the spring and summer months in the Australian school calendar. In this article we present these findings and the reflections of Principals, literacy and numeracy leaders and coaches about these findings and their suggestions for action. We argue that reforming school practices during Terms 1 and 4 and developing a deeper understanding of students' out-of-school learning and knowledge are essential for enhancing growth in achievement from September to March and for narrowing the achievement gap between marginalised and advantaged students. Further research of this phenomenon in the Australian context is needed.