The European Union objectives in the Horizon 2020 program aim to reduce environmental impact through strategies such as the improvement of energy efficiency and the use of renewable technologies. With regard to the goal of sustainable development—which integrates environmental, social, and economic dimensions relating to the preservation of the planet and the integrity of consumers—several types of sustainability certification tools are currently used in the construction industry e.g., Leadership in Energy and Environmental Design (LEED), the Building Research Establishment Environmental Assessment Method (BREEAM), and the Sustainable Building Tool (SBTool) There are international sustainability methodologies designed specifically for high schools and methodologies specific to the Portuguese reality, such as Natura Domus, LiderA (Liderar pelo Ambiente para a construção sustentável, Leading the Environment for Sustainable Construction) and SBtoolPT (Sustainable Building Tool, Portugal). Currently, with the concern in Portugal to requalify schools, it has become necessary to develop a specific methodology for school buildings according to the Portuguese reality. This work highlights the SBTool methodology that is employed in several countries and can be adapted to basic education institutions as the basis for the formulation of responsible citizens and the development of a country. The main aim of this study is to adapt an already existing assessment tool of sustainability (SBToolPT), maintaining some indicators while modifying and adding others, in order to develop a methodology specifically for conducting a Sustainability Assessment of High School Buildings in Portugal—SAHSBPT. In order to achieve this goal, other methodologies that already incorporate parameters relating to the school environment are analyzed, such as LEED BD + C Schools (LEED Building Design and Construction School), BREEAM Education 2008, and the SBTool for K–12 schools.