The importance of education, and ESD in particular, for achieving sustainable development is highlighted in the formulation of the Sustainable Development Goals (SDGs). Since the Brundtland Report (1987) and the Agenda 21 conference in Rio in 1992, many measures and programs have been launched. However, no widely accepted and validated assessment instruments are currently available to examine the output levels of ESD on the student side as a means to contribute to monitoring the effects of ESD initiatives. Furthermore, connections to the results of empirical educational research are often lacking. Indeed, operationalization is necessary in order to evaluate actions of fostering ESD. Taking concepts of empirical educational and other relevant research findings (for example, psychology for sustainability) into account, this study develops a reliable and valid approach to measuring sustainability competencies. In this paper, novel data of a first school assessment is presented. One thousand six hundred and twenty-two students (aged from 9 to 16) participated in the survey. The paper-pencil questionnaire covers general (socio-demographic) as well as cognitive, affective, behavioral, application- and curriculum-orientated aspects of sustainability competencies. The evidence for the validity and reliability of the instrument indicates that the presented assessment tool constitutes a suitable instrument by which to measure sustainability competencies in secondary schools. The gathered insights show a path towards the operationalization of sustainability competencies to clarify the needs and achievements of ESD implementation in schools.