How to promote social emotional learning (SEL) at school depends largely on teachers. Mostly teachers implement specific programs, but they have difficulties in incorporating SEL into the regular curriculum. The main aim of the paper is to present the conceptual model of sustainable integration of SEL into everyday teaching practices in every subject. This approach has been developed in the project "Learning to Be: Development of Practices and Methodologies for Assessing Social, Emotional and Health Skills within Education Systems". This initiative is based on the premise that the assessment of learning at school should go beyond grading students' knowledge and should include practices for observing young people's personal growth, social skills, attitudes and other general competences. The novelty of this conceptual approach is associated with integrating SEL standards, formative assessment and classroom instruction into a single sustainability-oriented model. The relationship between SEL standards (ISBE, 2003) and formative assessment strategies established by Wiliam (2011) is described, providing a detailed description of specific classroom activities. The objective of this approach is, therefore, towards building emotionally strong and flexible individuals who can deal with complex challenges through prosocial behavior that encourages human prospering and the attainment of the United Nations' Sustainable Development Goals.