Currently there is intense research activity to improve energy efficiency in buildings. School buildings are among the most numerous of public buildings in Europe, most of them built several decades ago. This paper analyses the inclusion of good energy efficiency practices within a school buildings renovation and modernization programme as an opportunity to improve their energy performance, with a reduced cost of upgrading the buildings. It is developed within the framework of a regional programme to renovate the stock of school buildings in the South of Spain mild climate zone, which amount to more than 4500 schools, as representative of mild Mediterranean climate area with approximately 87,000 schools. It studies the impact of a set of affordable, passive refurbishment solutions to improve the envelope of school buildings (insulation, shading and glazing) in a reference baseline building model. The results show relatively low energy demands compared with those reported in equivalent schools in other climates, 19.6 kWh/m2 for cooling and 11.1 kWh/m2 for heating. A high potential for energy efficiency improvement, with savings up to 17.7% for heating and up to 15.9% for cooling is obtained by combining affordable passive actions with an investment cost of 146 €/m2. •Passive refurbishment actions to improve Mediterranean school buildings efficiency.•A catalogue of actions on the building envelope analysed over a school design widely repeated.•Potential savings percentage up to 17.7 for heating and up to 15.9 for cooling.•Better improvement measures combination investment cost of 146 euros per square meter and execution in two months.•Potential carbon dioxide emissions savings of 9.5 tons per year and building.