The Illapel earthquake struck Chile in 2015, causing fatalities, wounded, missing, and many people left homeless. It also damaged buildings, particularly school buildings. This research refers to the “San Rafael de Rozas” school, located in Illapel (Chile). It was visited after the event, and a simplified OMA (Operational Modal Analysis) of the damaged building was performed; it is required to estimate the modal parameters (mainly, natural periods) of the damaged building. This three-story building was built in 2003 and has an L-plan configuration. The structure consists of RC columns, beams, and slabs; in general, the construction quality was bad. The non-structural components significantly influenced the seismic behavior, chiefly the masonry infill walls. Noticeably, most of them are incomplete (i.e., leave a free space in the top, mainly for lighting); this leads to short columns, which are harmful to the building's seismic capacity. Accordingly, the damage was mainly concentrated in the short columns. For the abovementioned OMA, triaxial acceleration and velocity-sensitive seismographs were installed on the floors of the 3 stories; each measurement lasted 6 minutes at a sampling frequency of 512 Hz.