There is much potential for supporting collaborative learning with interactive computer simulations in formal education and professional training. A number have been developed for single user and remote interaction. In contrast, our research is concerned with how such learning activities can be designed to fit into co-located large group settings, such as whole classrooms. This paper reports on the iterative design process and two in-the-wild evaluations of the 4Decades game, which was developed for a whole classroom of students to engage with a climate simulation. The system allows students to play and change the rules of the simulation, thereby enabling them to be actively engaged at different levels. The notion of Contributory Simulations is proposed as an instructional model that empowers groups to make informed, critical changes to the underlying scientific model. We discuss how large-group collaboration was supported through constraining an ecology of shared devices and public displays.