Computational Thinking (CT) is gaining a lot of attention in education. In this study we focus on the CT aspect modeling and simulation. We conducted a case study analyzing the projects of 12th grade high school informatics students in which they made models and ran simulations of phenomena from other disciplines. We constructed an analytic framework based on literature about modeling and analyzed students´ project documentation, recordings of student groups at work and during presentations, survey results and interviews with individual students. We examined how to discern the elements of our framework in the students´ work. Moreover, we determined which data sources are suitable for observing students´ learning. Finally, we investigated what difficulties students encounter while working on modeling and simulation projects. Our findings result in an operational definition of modeling and simulation, and provide input for future development of both assessment instruments and instructional strategies.