This book provides a unique analysis and description of the linguistic challenges faced by school students as they move from primary to secondary school, a major transition, which some students struggle with emotionally and academically. The study: • draws on a bespoke corpus of 2.5 million words of written materials and transcribed classroom recordings, provided by the project's partner schools; • combines quantitative and qualitative approaches to the corpus data to explore linguistic variation across school levels, registers and subjects; • describes the procedures of corpus compilation and analysis of written and spoken academic language, showing how modern corpus tools can be applied to this far-reaching social and educational issue; • uncovers differences and similarities between the academic language that school children are exposed to at primary and secondary school, contrasting this against the backdrop of the non-academic language that they encounter outside school. This book is important reading for advanced students and researchers in corpus linguistics, applied linguistics and teacher education. It carries implications for policymakers and schools looking to support students at this critical point in their schooling.