Not all MO send their kids to MO schools…some send them to charedi schools, Chabad schools, community schools that are mainly non-frum, even public schools.
Such choices may make it more likely that they will become not-MO when they grow up, but when they’re still kids and living with their MO parents, they’re still MO.
Anyway, a lot of people have no choice, because many MO live in a place with no MO school, because there’s not enough MO to have their own separate school.