The lexicon, a complex storage device of units of language use, is a central component of linguistic knowledge closely tied to grammar and has a strong influence on demanding cognitive tasks and academic achievement. This study aims at tracking the growth of the Catalan written lexicon of children and adolescents throughout compulsory schooling, a time when the lexicon is assumed to experience an exponential growth. 2,436 participants from 5 to 16 years attending compulsory school in Catalonia took part in the study. They were asked to write as many linguistic expressions as possible in five different semantic fields: Food, Clothing, Leisure activities, Personality traits, and Natural phenomena. Although both the task and the provided examples primed the production of single words, participants produced a variety of constructions. The 242,404 lexical forms that were produced were lemmatized into 8,498 different lemmas and coded according to different linguistic dimensions. The size and the conceptual underpinning of the lexicon grow significantly throughout compulsory school and show an increase in the use of correct Catalan forms, a reduction in deviant forms, and a steady use of words and constructions in other languages. The use of multiword constructions as a mechanism for word generation questions the separability between lexicon and syntax. The corpus, which is of public access (http://clic.ub.edu/es/cesca), provides a picture of the state of a language developing in a multilingual environment in terms of frequency of use of words and constructions and range of orthographic and linguistic variants in five semantic fields.