This teacher-researcher/university-researcher collaboration focuses on teaching and learning in untracked English classes, but has implications for all classrooms where students have different needs. We primarily examine the teacher-researcher's (Delp's) eighth-grade untracked English class but also include data from a group of beginning teachers learning to teach in similar settings. In the end, we challenge previous findings about teaching and learning in untracked English classes and raise issues about the theories that guide such work as well as the relationship between theory and practice. In the context of strong student growth across achievement levels and ethnic groups, we found that the activity system in Delp's classroom differed markedly from what is usually recommended for teaching heterogeneous groups. Instead of teacher-organized small groups (as in cooperative learning or complex instruction), Delp relied on whole-group, multimodal activities and one-on-one teacher-student interactions during group activities. We hypothesize that the activity system is not critical in deciding how best to teach in untracked classrooms. More important is a set of underlying principles, rooted in Vygotskian and Bakhtinian theory, which support the activity system. The principles include (a) building a long-term curriculum that promotes the recycling of structures and ideas, with room for ever-deepening levels of complexity; (b) considering learners to be in control of their learning and building structures that support them in challenging themselves; (c) building a learning community that respects and makes productive use of diverse contributions from varied learners; (d) providing opportunities for diverse ways of learning; (e) providing support to individuals as needed; (f) challenging all students; (g) keeping learners actively involved. This analysis of Delp's teaching and her students' learning further shows how Vygotsky's "zone of proximal development" functions within a whole-class context, where students are involved at their different levels and where the teacher plays an active role in tailoring instruction to these levels, both through the whole-class activity system and through the ongoing one-on-one interactions. Finally, the analysis explores how notions of development across long stretches of time can help teachers see and understand growth for an academically and socioculturally diverse group of adolescent students, growth that includes intellectual as well as ethical and emotional components. (Contains 11 tables, 2 figures and 4 notes.)