The projection of enrollment changes is vital to the planning of educational facilities. However, existing methods of projecting urban populations fall short of acceptable performance because they fail to adequately take into account residential mobility. As a starting point for improving these methods, this report develops an accounting model of student flows in a large urban school system. The model is used to identify components of change in grade level enrollment for each school and to suggest research designs for investigating processes generating these changes. The model is also intended to be used as a basis for carrying out empirical analyses of student flows in large school systems and for developing improved techniques of projection. (Author/MLF)