* Varying definitions of "school": This includes everything from preschools and kindergartens to primary, secondary, and tertiary institutions (colleges and universities). The lines are often blurred.
* Lack of universal reporting: Many countries lack comprehensive, regularly updated educational statistics. Especially in developing nations, data may be incomplete or unreliable.
* Informal education: Many children receive education outside of formal school systems. These are difficult to account for.
While various organizations attempt to estimate the number, these are broad approximations, often with considerable margins of error. You'll find widely varying figures depending on the source and their methodology.