Presumably the police were able to determine that it was not a genuine explosive device more or less immediately upon arrival. Indeed, it seems likely that the teachers made that determination by themselves. Or, to be exact, the media reports (usually error-filled, lazy, and sloppy, to be sure) do not report that the teachers put the alleged "bomb" out in the middle of the field, or evacuate the school, or do any of the things you'd normally do with a suspected bomb.
If we stipulate (in spite of the sloppiness of the media) that everyone knew, or was fairly certain, early on that it was a bomb, we're now in the land of "well, I think the kid made a hoax bomb which is against the rules" and now we're arresting the kid based on what we think his motivation might have been. In the absence of any obvious "tells" like fake sticks of dynamite, or globs of claylike substances, or similars, we're arresting him based on what we presume the contents of his mind to be.
We are, in short, in the world of arresting people for thought crimes.
That's not a good place to be.