This paper presents a seismic vulnerability assessment of a complex reinforced concrete (RC) school building constructed in the mid-1970s. The structure was originally designed to support vertical loads only, predating the extension of seismic hazard classification to nearly the entire Italian territory. The case study is located within a school campus in Pomigliano d’Arco (Naples, Southern Italy). The building features a quadrangular plan layout and extends over four above-ground levels, reaching a maximum height of approximately 14 meters. It is organized around a large central covered atrium, approximately 30 × 22 meters in size, spanned by a steel roof structure composed of trusses and a corrugated metal deck or other metallic material. The atrium serves as the connection point between the various volumes that compose the building complex. The load-bearing system consists of ordinary RC frames (beams and columns), mainly oriented transversely to the longitudinal axis of each block. The floors are made of clay–concrete composite slabs on all levels, including the roof, except over the atrium, where a steel structure is used. Despite its large plan dimensions, the building lacks seismic joints, resulting in unified structural behavior. However, for the purposes of seismic modelling and analysis, a functional subdivision into four idealized blocks was introduced, with separations corresponding to the RC walkways that link the various volumes. This modelling strategy enables a more effective structural interpretation and clearer identification of potential local vulnerabilities.