Many reactive control systems consist of classes of active objects involving both intra-class interactions (i.e. objects belonging to the same class interacting with each other) and inter-class interactions. Such reactive control systems appear in domains such as telecommunication, transportation and avionics. In this paper, we propose a modeling and simulation technique for interacting process classes. Our modeling style uses standard notations to capture behavior. In particular, the control flow of a process class is captured by a labeled transition system, unit interactions between process objects are described as transactions and the structural relations are captured via class diagrams. The key feature of our approach is that our execution semantics leads to an abstract simulation technique which involves: (i) grouping together active objects into equivalence classes according their potential futures, and (ii) keeping track of the number of objects in an equivalence class rather than their identities. Our simulation strategy is both time and memory efficient and we demonstrate this on well-studied non-trivial examples of reactive systems. We also present a case study involving a weather-update controller from NASA to demonstrate the use of our simulator for debugging realistic designs.