Research on stratification in schools shows that the differentiated structure of schools is associated with increasing inequality of educational outcomes. Some writers have therefore advocated "detracking"--the elimination of stratification within schools. Other writers have suggested that differentiation within schools is possible without the harmful effects of tracking. This paper examines 24 highly restructured schools, using qualitative and quantitative data collected at the Center on Organization and Restructuring of Schools at the University of Wisconsin-Madison. The data show, first, that detracking is a goal that is idealized more often than it is achieved. Detracking appears to face more barriers in secondary than in elementary schools, and it is resisted in math more than in other subjects. Close analysis of two cases--one high school and one middle school--show that neither heterogeneous nor homogeneous grouping present insurmountable barriers to high-quality instruction; however, neither approach ensures effective teaching. Conditions that supported high-quality instruction in a mixed-ability context included small class sizes, extra resources that permit a highly individualized instructional approach, strong intellectual leadership, and selection of teachers and students. Conditions that supported effective instruction in the context of differentiation included a commitment to equity across classes, teacher and student selection of course, and teachers' intellectual commitment to the subject matter. One table is included. (Contains 23 references.) (Author/LMI)