This study proposes an advanced methodology for the seismic retrofit of reinforced concrete (RC) structures using dissipative exoskeletons, designed through a performance-spectra-based approach. Exoskeleton systems—external, steel-framed structures rigidly connected to the host building—represent an effective, reversible, and low-impact retrofit solution, especially suitable for operationally critical facilities such as schools and public buildings. Unlike conventional retrofitting methods that often rely on iterative trial-and-error procedures, the proposed strategy employs Performance Spectra (P-Spectra) to establish a direct relationship between desired structural performance levels and the mechanical properties of the dissipative devices. By aligning the seismic response of the combined system with performance targets expressed in terms of displacement and acceleration response factors, the procedure enables precise sizing of the exoskeleton components, including braces and dampers. The study demonstrates the application of this approach to a real-world RC school building in southern Italy, retrofitted with eccentric braced frames and steel strip dampers. Nonlinear time-history analyses, based on a suite of spectrum-compatible ground motions, confirm the method’s effectiveness in reducing inter-story drifts and peak accelerations, while ensuring yielding of all energy dissipation devices. The results highlight the P-Spectra method as a reliable and efficient design tool, capable of streamlining retrofit processes and enhancing both structural safety and sustainability. By minimizing internal disruption and aligning with contemporary circular economy goals, the proposed framework represents a promising path toward resilient and adaptable seismic rehabilitation in densely built environments.