Toaru Majutsu no Index - Volume 8, Chapter 2: Girls facing off. Space_and_Point.

... the financial power that each school had, having their own buses wasn’t a problem. However, each school had the ideal of ‘letting students engage society more for their own safety’, and gathered the bus system as one.

The buses owned by the five schools had large and luxurious interiors, earning it the moniker ‘double procession limo’. The student seats were located below, while the top was a café. The bus would always move in a fixed route, down the roads to the five dormitories.