That’s for a fairly broad definition of “migration background”. Everyone who wasn’t born a German citizen or has at least one parent who wasn’t a German citizen

That’s for a fairly broad definition of “migration background”. Everyone who wasn’t born a German citizen or has at least one parent who wasn’t a German citizen at their birth qualifies (“Wer die deutsche Staatsangehörigkeit nicht durch Geburt besitzt oder mindestens einen Elternteil hat, bei dem das der Fall ist, hat einen "Migrationshintergrund".”) For example, Angela Merkel’s mother was born in Danzig in 1928. That wasn’t part of Germany at the time (https://en.wikipedia.org/wiki/Free_City_of_Danzig https://en.wikipedia.org/wiki/Free_City_of_Danzig), so chances are she wasn’t born with German citizenship. If so, Angela Merkel has a migration background according to that definition. For primary school kids, I would guess many of them are children of people who have lived and worked in Germany for decades.