This case study grew from the researcher's interest in whether the middle school context was appropriately matched to the developmental needs of young adolescents and her desire to understand why many students in middle school begin a decline in academic achievement, lose motivation, and become disengaged from school. Eccles and colleagues' (Eccles, Lord, & Midgley,1991; Eccles, Lord, & Roeser, 1996; Eccles & Midgley, 1989; Eccles & Wigfield, 2002; Wigfield & Eccles, 2006) advanced stage-environment fit theory for their studies of adolescent schooling. Their research findings confirmed that the educational environment in middle school did not match the developmental needs of young adolescents and therefore created negative outcomes. The purpose of this study was to research middle school teachers' knowledge of the developmental characteristics of young adolescents and investigate how the teachers aligned this knowledge in the classroom. Did the teachers create classrooms and instructional practices that fit young adolescents' developmental needs, including their physical, emotional/psychological, moral, social, and intellectual/cognitive needs? Were there differences in teacher knowledge, instructional practices, and classroom environments between a high performing middle school and a low performing middle school? Using qualitative research methodology, the researcher examined teacher knowledge and teacher practice in two urban middle schools from the same California district, one high-performing and one low-performing. Both schools served 6th through 8th grade students, and teachers at both schools were organized into interdisciplinary teams. The researcher surveyed eight teachers, four from the low-performing school and four from the high-performing school and conducted classroom observations in each of the teacher's classrooms. Artifacts of teacher lesson plans, student documents, and data from schools websites were collected and analyzed. Pseudonyms were used to describe the district, schools and teachers. The Middle School Teacher Survey, developed for this study, revealed that all eight teachers had "Partial Knowledge" of the characteristics of young adolescents; however, the depth of their understanding varied. During classroom observations the researcher noted differences in classroom environments and in instructional strategies used by the teachers. In contract to instruction in the low performing schools, teachers in the high-performing schools offered students opportunities for autonomy, high-level thinking activities, and a variety of instructional strategies that actively engaged students. Findings confirmed the need to provide both inservice training in the area of adolescent development for current middle school teachers and additional learning opportunities for pre-service teachers who wish to teach in middle schools. Findings from this study have practical implications for teachers' practice and their students' motivation, engagement, and academic achievement. Recommendations for future research include using a more precise measurement tool to assess teacher knowledge and studying a larger population.