To me the principal is not at fault here.
The police on the other hand should know better.
They should have told the principal that they can not arrest the kid for making non-specific threats.
Then the police should have councilled the kid in a private room that making such statements will bring him undesirable attention.
Then his parents should have been notified of his stupid behavior.
I would think that the police are at fault here and not the principal. Although the principal did act stupidly.
No, all are at fault. The principal called the police. The cops come to these things dying to make an arrest to show how important they are. There is no reason for this other than the egos of all involved.