There are various retrofitting methods for reinforced-concrete (RC) buildings. Most of them can be verified only by numerical analysis and component test in the laboratory. However, the actual efficiency of those retrofitting methods cannot be verified well. Performing a field test with the actual school building is the most direct way to verify those methods. In-situ push-over experiments and analytical seismic-resistance assessments were conducted in this paper to investigate the seismic capacity of RC school buildings at the Guan-miao Elementary School in Tainan City, Taiwan. Both prototype and external steel-framing specimens were constructed and compared. Reinforcements with steel channel jacketing were adhered to the captive columns and their adjacent beams in the weak direction to retrofit the prototype school building through the creation of external steel-framing systems. The design and construction procedures of beam-column connection and column-base anchorage detail drawings were introduced to ensure the effect of proposed retrofitting method. Experimental results show that the steel-framing systems are firmly bonded to the retrofitted specimen, providing continuous stress transference from the beam-column joints to the steel channels. The failure mode of the retrofitted captive columns was shifted from shear failure of the prototype specimen to flexural-shear failure as designed. The proposed external steel-framing systems effectively enhance the base shear strength and the ductility of the specimens, indicating that it is indeed a feasible method for retrofitting construction of RC school buildings.