If it was meant as a joke, then it was a particularly sick and cruel one. One can only imagine the sense of alarm that would have spread through the school community when word of the "threat" go out.
If it was done out of malice, then it is especially contemptible.
Either way, it would be nice to see the hoaxer revealed and made to face the consequences of his or her sick actions.
The one "plus" may be that the school got to test-run its emergency procedures and seems to have handled the situation very well.
And finding that the police response was swift and thorough will be a consoling thought for all.