* Age group: School transitions are much more common for younger children (starting kindergarten, changing elementary schools) and teenagers (moving for high school or college).
* Country: Educational systems, family mobility, and economic factors differ significantly between countries.
* Definition of "change school": Does this include moving to a different classroom within the same school, transferring between schools in the same district, or only moving to a completely different district or even country?
To get an estimate, you'd need to specify these factors and research the relevant statistics for the specific region and age group you're interested in. You might find data from education ministries or statistical agencies within individual countries, but a global figure is unlikely to exist.