The Italian secondary education system shows difficulties in introducing the paradigm of sustainable education. Its curricula convey good contents to the students and allows to train specialized and technically competent teachers which, however, are not properly prepared to strengthen critical thinking and aware acting in students. The project “School 21”, proposed since 2009 by Fondazione Cariplo (Milan, Italy) to the secondary schools and the vocational training centers, aims to improve the italian secondary degree curricula including in the annual planning an interdisciplinary teaching methodology focused on some main sustainability issues: biodiversity, sustainable energy, environmental quality and climate change allow to link articulated and apparently isolated elements, phenomena and situations proper of the various fields of knowledge. The students are facilitated to explore in a systemic and metacognitive way the disciplinary contents emerging from the reality and are directly involved in planning and implementing a solution to a local problem. The learning process supported by the project is centered on the definition of a detailed learning plan which constitutes the essential instrument to achieve the objectives of the project; it allows the teachers to link the students’ skills with the disciplinary subjects, the educational methods, the teaching aids and the expected outputs from the students. A set of quality criteria, defined during a preliminary participated research which involved different stakeholders (experts, teachers and students), orientates and inspires the teachers in the planning of the training process. In addition, also the assessing process of the citizenship skills attained by the students is improved, as requested by the Italian Ministry of Education; a system of indicators has been specifically defined during the preliminary participated research. The students’ and teachers’ remarks collected at the end of the first implementation of the project (school years 2011/2012 and 2012/2013) show the achievement of the objectives of the action research process put in place and demonstrate the effectiveness of the paradigm of sustainable education for the improvement of educational processes in Italian secondary school.