Learner diversity in foreign language classrooms is often handled as a placement problem. Students are grouped by entrance or proficiency scores, and differentiated instruction is expected to follow from the resulting class structure. This critical integrative review questions that assumption and develops a framework for tiered task design in mixed-ability foreign language classrooms. Drawing on research on ability grouping, tracking, differentiated instruction, second language motivation, task-based language teaching, dynamic assessment, and recent AI-mediated EFL support, the article argues that the central issue is not whether learners differ, but how those differences are identified, interpreted, adjusted for, and made consequential in classroom life. Evidence from general education offers limited support for long-term fixed between-class grouping and points to risks of labeling, reduced teacher expectations, weakened academic self-concept, and unequal access to demanding tasks. Evidence from differentiated instruction and EFL task studies is more favorable to within-class adjustment, although it remains fragmented and does not yet establish tiered task design as a superior intervention. On this basis, the review proposes a four-dimensional account of learner-diversity organization and specifies four mechanisms for tiered task design: shared communicative goals, graded scaffolding, scaffolded choice, and developmental assessment. The framework is presented as a researchable pedagogical model rather than a settled solution, with implications for language programs seeking to support participation and equity without turning proficiency differences into fixed learner labels.