In this paper we describe the design of an online classroom-based simulation, ClassSim, developed to enable pre-service teachers to interact with a virtual classroom environment as they assume the role of the teacher. The project built on the research of Herrington and Oliver by investigating how the design elements of authentic learning environments they identified can be operationalised in an on-line simulation of a classroom. The aim of the simulations was to progressively develop preservice teachers' understanding of the complex decisions needed to plan and implement reading, writing and spelling lessons for lower primary children. With the support of a large grant from the Australian Research Council entitled: Investigating a classroom simulation designed to support pre-service teacher decision making in planning and implementing literacy teaching (DP0344011) we have engaged in iterative designs and accompanying trials of the software with more than 500 pre-service teachers studying within the Faculty of Education at the University of Wollongong, Australia. During these trials we noticed some trends in the ways in which pre-service teachers approached the software and report on the implications of these trends for instructional designers.