They aren't the customers, though, not even officially. If anyone's the customer of K-12 schools, it's the adults of society who believe the schools should exist, and should educate kids in a particular way (and who also pay for the schools' upkeep). That's why kids don't get to choose their curriculum, for example; adults decide what should be taught in schools. Kids aren't allowed to opt-out, either (they can be arrested for truancy if they try).

Yes, I'm very aware of truancy laws. Perhaps I should have said supposed beneficiaries.