We ought to pay attention to serious terroristic threats. We all understand that such remarks are merely rhetorical. The student's remarks fell into that same category, and he ought not to have been punished for them. The administrators also showed a profound ignorance of more than rhetoric, though, when they expelled the student. They also showed a lack of respect for the privacy that guarantees true freedom of thought.
Decent people allow others a zone of privacy.
That zone includes what we think, what we say to ourselves, and what we say to people with whom we maintain relations of an intimate and private nature. What people say in private communications with friends is special and privileged, and the rhetoric of conversations between friends is nobody else's business. Both the chaperone who printed out this private conversation and the administrators who received it violated this privileged zone. The chaperone's reading of private mail was offensive enough, but the administrators compounded the affront by acting on it as they did. You would think that adults would know better.