The main aim of this research is to present an evaluation tool for the Network of Schools for Sustainability in Catalonia, which comprises 18 networks with more than 1400 schools. The secondary aim is to reflect on the construction process conducted. The tool was constructed following the tenets of constructivist evaluation by Guba and Lincoln, which is characterized by the engagement of multiple voices from a range of stakeholders, using a consecutive hermeneutic dialectical process, understood as a negotiation among participants. To ensure these processes, in the construction of the tool the fourth authenticity evaluation criteria were followed. The main outcome of the project is an evaluation tool composed of 25 criteria which are divided into 5 dimensions: Structure of the Network, Training Offering, Collaboration with External Stakeholders, Guidance of the Networks and Communication. Some transversal areas could be seen in the tool, such as the role of evaluation and the involvement of stakeholders from the school, which represent areas of special relevance for the participants in the process.