* Geographic location: The number of school-aged children and school attendance rates differ significantly between countries and even within regions of a country.
* Age range considered: Are we talking about preschool, primary, secondary, or all levels of education?
* Definition of "school": Does this include homeschooling, alternative education programs, or only traditional brick-and-mortar schools?
To get an accurate number, you'd need to specify the above parameters and consult data from relevant organizations like UNESCO (for global statistics) or individual national education ministries.