A project named PERSISTAH is being developed to study the seismic vulnerability of primary schools in Huelva (Spain) and the Algarve (Portugal). This area has a moderate seismicity but this is affected by a nearby area where earthquakes of large magnitude (Mw≥6) and longreturn periods happen. The seismic vulnerability of URM (UnReinforced Masonry) buildings has been observed and analysed in the last decades. The seismic retrofitting of these buildings is required in order to improve their seismic behaviour. Many retrofitting techniques have been developed for that purpose, most of them very complicated and expensive. Therefore, these are not appropriate to retrofit a large number of buildings. This is especially relevant in areas of moderate seismicity where the cost-efficiency ratio must be carefully considered. The aim of this paper has been to develop a simple, effective and affordable technique to retrofit these buildings. These buildings are characterised by numerous openings which causes a great weakness in the URM walls. Then, a technique that consists in installing a steel encirclement or a grille in the openings of the walls has been proposed. This is a specific retrofitting technique for URM walls since this technique substantially improves the seismic capacity of these structures. To test the technique a case study is proposed. The building under study is a primary school located in Huelva and built in 1961. Results have shown that the capacity of the building is notably increased. Also, the performance point and the damage level of the structure are decreased.