As young people progress from primary school, through secondary, and, for many, on to university, they gain knowledge, develop skills, and learn to relate to others in ways that remain influential throughout their lives. Language is central to this process. At each educational stage, students need to understand and express new types of content to new audiences in new contexts. For some, this offers a rich opportunity for language growth. For many, however, academic language is a barrier that can seriously hamper their educational and life chances. Understanding language development at school and university is therefore central both to understanding linguistic, and sociocognitive development and to achieving a socially just education. This chapter reviews the contribution that corpus research has made and can make to understanding educational writing. It reviews the main foci of research, critically evaluates its methods, summarizes our current state of knowledge, and suggests directions for future work. It then looks in detail at one project, led by the author, that researched the grammar and vocabulary of children in schools in England. It highlights central findings and reflects on methodological strengths and weaknesses and on what lessons might be learned for future research.

This chapter delves into the critical role of lexis in L1 and L2 acquisition, exploring how vocabulary reflects language development and impacts text quality. The mental lexicon forms an intricate web of semantic connections, with bilinguals processing words differently based on proficiency. Low-proficiency learners rely on L1 translation, while advanced speakers strengthen direct links to L2 vocabulary. Research shows that both languages remain active during lexical tasks, shaping bilingual cognition.

Lexical richness is analysed through key models, including Crossley’s and Jarvis’. Advances in natural language processing have enabled automated evaluation tools like Coh-Metrix and TAALES, enhancing lexical analysis.

As bilinguals progress, their writing becomes more diverse and sophisticated, though L1 and L2 development may diverge in features like word concreteness. Formulaic language is also crucial – high-proficiency L2 writers use more native-like phrasal structures. By examining lexical acquisition, this chapter highlights its significance in bilingual proficiency, providing insights into how vocabulary shapes linguistic competence.