How upper secondary education is organized shapes the duration of common learning, the point at which specialization begins, and students’ ability to combine or reconsider their choices. This study compares the curriculum structures of Iran, Finland, Germany, and England and identifies design principles for reorganizing Iran’s general upper secondary curriculum toward a comprehensive diploma. A qualitative comparative multiple-case design was used, drawing on Bereday’s four stages of description, interpretation, juxtaposition, and comparison, alongside document and framework analysis. The cases were selected purposively to represent contrasting arrangements of centralization, specialization, and student choice. The corpus included 23 current official curriculum, regulatory, assessment, and certification documents available up to July 2026. Four configurations emerged: a centralized fixed-stream structure in Iran, a modular-integrative structure in Finland, a domain-balanced course structure in Germany, and a qualification-selective structure in England. Key differences concerned the common curriculum, timing of specialization, pathway permeability, local authority, guidance, and the relationship between school learning and external certification. On this basis, a core-concentration-choice framework is proposed for Iran, combining a sustained common curriculum, gradual concentration, guided electives, bridge modules, continuous guidance, interdisciplinary work, and multi-source assessment. A comprehensive diploma, the comparison suggests, need not eliminate specialization; it should make common learning, focused study, and revisable choice work together more effectively.