Background/Context: When new, rigorous standards are adopted, teachers often need to learn new content and new ways of teaching while concurrently attending to accountability demands. Both formal and informal school structures potentially enable this new learning, and school leaders likely influence the nature of these structures. Purpose/Objective/Research Question/Focus of Study: We examine teachers' learning opportunities in one school by asking the following research questions: (1) What is the nature of changes in teachers' formal learning opportunities, as seen by changes in teachers' workgroup conversations about mathematics instruction? (2) In what ways do school leaders shape the nature of instructional conversations, and thus formal learning opportunities, in teacher workgroups? (3) What is the nature of changes in teachers' informal opportunities to learn, as seen by shifts in informal advice networks? Research Design: This is a longitudinal case study using mixed methods: qualitative analysis of audio-recorded teacher workgroup meetings and quantitative analysis of informal social networks. Data Collection and Analysis: This analysis is a part of a larger eight-year longitudinal study, the Middle-school Mathematics and the Institutional Setting of Teaching (MIST). Data used in this analysis were collected over a three-year period in one middle school that was working to improve mathematics instruction by focusing on teaching mathematics conceptually and building procedural fluency. Data used in this analysis include audio-recorded teacher workgroup meetings, informal social network surveys, interview transcripts, and student-level standardized test scores. Findings/Results: We found that formally, school leaders shifted teachers' workgroup conversations away from instructional matters to those of standardized tests. Informally, teachers stopped going to each other for instructional advice. Triangulating interview data confirmed that over time, pressure that teachers felt to do well on the standardized tests shifted their attention away from a conceptual approach to instruction and toward an emphasis on test preparation. Conclusions/Recommendations: Our findings suggest that school leaders must be involved in new learning about standards and instruction to appropriately support teachers' learning opportunities.