Learning context has increasingly been postulated to exert an influence on the dynamics of individual differences in language learning. In a longitudinal design that tested the proficiency gains of 1,682 learners over a 2-year foreign language program, a multilevel modeling approach was deployed in this study to account for variation in second language (L2) growth over time. Different aspects of learner career and social aspiration were postulated as individual difference factors in addition to the more conventional constructs of extrinsic and intrinsic L2 learning motivation. Perceived attitudinal norms operating within 72 streamed classes were posited as contextual factors. Results of multilevel analyses indicate that the mediating effects of class context on the relation between the L2 proficiency growth and individual difference factors exert both positive and negative influences, suggesting malleability of individual differences factors.