As states and districts implement more rigorous teacher evaluation systems, measures of teacher performance are increasingly being used to support instruction and inform retention decisions. Classroom observations take a central role in these systems, accounting for the majority of teacher ratings upon which accountability decisions are based. Using data from the Measures of Effective Teaching study, we explore the extent to which classroom composition influences measured teacher performance based on classroom observation scores. The context in which teachers work--most notably, the incoming academic performance of their students--plays a critical role in determining teachers' measured performance. Furthermore, the intentional sorting of teachers to students has a significant influence on measured performance. Implications for high-stakes teacher accountability policies are discussed.