The paper presents the Šolar developmental corpus of Slovene, comprising the written language production of students in Slovene elementary and secondary schools, along with teacher feedback. The corpus consists of 5485 texts (1,635,407 words) and includes linguistically categorized teacher corrections, making the corpus unique in reflecting authentic classroom correction practices. The paper addresses the corpus compilation, content and format, annotation, availability, and its applicative value. While learner corpora are abundant, developmental corpora are less common. The paper bridges the gap by introducing the evolution from Šolar 1.0 to 3.0, emphasizing improvements in text collection, error and correction annotation, and categorization methodology. It also underlines the challenges and unresolved issues of compiling developmental corpora, most notably the lack of openly available tools and standards for different steps of the compilation process. Overall, the Šolar corpus offers valuable insights into language learning and teaching, contributing to teacher training, empirical studies in applied linguistics, and natural language processing tasks.