* Data Collection Differences: Different countries have different methods (or lack thereof) for tracking school transportation usage.
* Defining "Bus": The definition of a "school bus" can vary. Does it include minibuses, vans used for school transport, or public transit buses?
* Rural vs. Urban Differences: Bus usage differs dramatically based on geographic location and population density.
* Accessibility and Affordability: Access to school bus services varies greatly depending on income levels and infrastructure.
While a precise number is impossible to obtain, it's safe to say that it is a very large number, likely in the hundreds of millions, considering the global population of school-aged children.