In Canada, the Early Development Instrument (EDI), a measure of school readiness, has contributed significantly to our understanding of how children are faring in the early years. The EDI measures school readiness on five domains: 1) physical health and well-being, 2) social competence, 3) emotional maturity, 4) language and cognitive development, and 5) communication skills and general knowledge. The EDI has been associated with elementary school outcomes however, there is a dearth of research examining the lingering effects of school readiness on school outcomes beyond elementary school, specifically in early adolescence. The objective of the study is to examine associations between school readiness in Kindergarten and three long-term school outcomes: Grade 7 mathematics, Grade 8 reading and writing, and Grade 7 student engagement. With linkable de-identified population administrative, registry, survey, and other databases housed at the Manitoba Centre for Health Policy, logistic regression analyses were used to examine the associations between the EDI in Kindergarten and Grade 7 and 8 school outcomes for a total of 16,252 youth, after adjusting for individual, family, and neighbourhood level variables. Children identified as ‘not ready’ on one or more EDI domains in Kindergarten had increased odds of doing poorly on all Grade 7 and 8 school outcomes. The associations between school readiness and all school outcomes did not differ by area-level socioeconomic status. An increasing number of ‘not ready’ EDI domains corresponded with increased odds of doing poorly in all school outcomes. Each EDI domain was significantly associated with Grade 7 and 8 school outcomes. This study underscores the importance of development in the early years and school readiness for long-term academic trajectories. The findings suggest that multidisciplinary and collaborative efforts should be aimed at ensuring children arrive in Kindergarten ready to benefit from school-based learning and to ensure that all children can thrive in the school environment and thus attain their many goals that would allow them to become responsible, productive and happy citizens.