A shift towards the mainstreaming of ELs presents new complexities for teachers' work with EL populations. This shift presents an opportunity to explore how teachers adapt and learn through teaching in a full-inclusion setting and their everyday interactions with colleagues. This qualitative study explores how high school social studies teachers leverage resources and the discourses one district and teachers engage in around developing EL instructional practice and their own teacher learning over time. In a full-inclusion context, I ask: How does context mediate teachers' learning as they navigate district policy shift towards the mainstreaming of ELs? This study draws on Activity Theory (Vygotsky, 1978; Engestrom, 2000; Greeno, 2006) as a situative learning approach to explore teacher learning over time and how district discourses and resources mediate teachers' opportunities to learn. Drawing on a cultural-historical activity theory perspective (Cole, 1996; Leont'ev, 1979) and specifically through an activity theory lens (Engestrom, 1987; 1999; Greeno, 2006), I consider the following tools in order to understand how contexts mediate teacher learning: Knowledge, principles, and practices (Lampert et al, 2013). This study includes aspects of qualitative design (Merriam, 2009; Patton, 2002), case study (Yin, 2009), and discourse analysis (Gee, 2011). I selected two comprehensive high schools within a large, suburban district, both similar in student demographics and EL demographics, but unique in the academic course offerings and other school specific elements. Data for this 6 month study includes multiple semi-structured interviews with high school social studies teachers, observations of classroom instruction that consisted of three day lesson sequences, teacher planning meetings and work meetings, classroom and teacher meeting artifacts, and teacher conversations and emails. Findings contribute to how we understand the role of context in mediating teacher learning, such as which classes teachers teach, the role of certain factors such as discourse and resources in a district setting, and to deepen understanding of the mainstreaming phenomenon of English learners at the secondary level, and the complexity and variation across EL student proficiency levels. Implications lead to how district EL policy can act as a catalyst to limit or afford learning opportunities for teacher learning and educational reform in the secondary school context.