This study analyses the effects of learning contexts on proficiency development as well as attitudinal and behavioral changes. At a Japanese high school where content-based L2 instruction in global studies is a feature of education, TOEFL scores, international posture, L2 WTC, and frequency of communication in L2 were assessed in the participants' first year and third year, and compared between (a) study abroad and stay-home groups, and (b) two program options with substantially different class hours and emphasis in education. The results indicate that the study abroad group demonstrates a clear advantage in all of the indicators over groups who stayed home. Subsequently, a cluster analysis delineated three clusters that show clearly distinct developmental patterns among those who stayed home. One of them exhibited a developmental profile similar to the study abroad group. This result implies that the development in proficiency and attitudinal and behavioral changes can take place when the learners fully participate in a community of practice of learners and teachers that links to an imagined international community. (Contains 5 figures and 6 tables.)