In the present study, we concentrate on the formulation and the exact solution of a specific vehicle routing problem with environmental concerns, which is a complex actual problem of staff service bus route planning for a university in a metropolitan city. Taking into account the air pollution related measures for representing the cost of environmental concerns in addition to the vehicle rental and fuel consumption related operational costs, we formulate a flow-based mixed integer linear program for the environment-friendly school bus routing problem specific to both homogeneous and heterogeneous vehicle fleets. We provide exact solutions to the instance sets we have designed specific to variants of the problem considering the fleet type, vehicle capacity, and maximum route length. Results from model based exact solutions for our formulations show up to 30% of reduction in the overall cost for the heterogeneous fleet case when compared to the homogeneous ones, and suggest a routing plan that enables the utilization of almost the entire capacity of all the buses assigned. We discuss in details the trade-offs between the cost items, travel times, and travel distances in composing a vehicle fleet considering the demand sprawled over the metropolitan area.