The present study investigated how initial proficiency impacted development in adolescent learners’ L2 speaking. The study reports on a longitudinal study with dense measurements with twelve adolescent L2 English learners. The participants were tested every week throughout their first year of secondary school (30 datapoints). The participants’ learning trajectory was modeled using generalized additive mixed models (GAMMs). Results showed large differences in development between various lexical and syntactic measures and between individual learners. The learners’ initial proficiency had a significant impact on their development. Overall, more periods of growth were observed in lexical measures than in syntactic measures. In line with previous results, some stabilization was observed once learners reached a certain proficiency level, but this stabilization was dependent on task type. Closed tasks lead to a ceiling effect in some measures, whereas more open tasks give learners from various proficiency levels opportunities to demonstrate their L2 English speaking skills.
