Imagine you have a bus coming every day at 7am. Now you move school's start time back 2 hours. You change the schedule so now the bus comes at 9am. What's the bus doing at 7am? Now that bus is sitting around not doing anything at 7am. Adding the second loop means you need to pay the bus driver for 2 extra hours and you need gas for another loop, but these costs are very small compared to what you are already paying to own and operate the bus and employ the bus driver. Assuming a 30 mile route with 80 students at 6 mpg, you're looking at less than 25 cents per student per day, about a 4.5% increase over the current average expenditure per student transported. This can be further reduced if you know which students will be riding in each time slot so you can take two shorter routes.