While numerous studies have focused on techniques that promote foreign language acquisition in an instructed context, most studies on the effectiveness of instructional techniques have been carried out in controlled lab-based settings. Data on how foreign language teaching is realized in the L2 classroom and how this affects L2 learning are scarce. In this project, we focus on the development of language proficiency in 16-18-year-old Flemish foreign language learners of French. By means of a cross-sectional study in which data are collected in three different years of secondary education, we want to investigate the development of language proficiency. A combination of questionnaires and observations will be used to gain insight into teachers' didactic practices and learner-related characteristics and to investigate how these variables impact the development of L2 language proficiency. This project will contribute to research on instructed SLA and language didactics and will have important pedagogical implications