Consequence of a litigious society. Here's a question for you: if this kid comes along later in life and beats the tar out of somebody, and in that lawsuit they point back to this incident and use the argument "Look! He's got a history of threatening other people. Doesn't matter that it wasn't a _credible_ threat. He threatened other kids and the school did nothing! Blame the school. Sue the principal and every teacher who ever saw the kid.", are _you_ going to spend all of the money and time to hire the lawyers to defend the school from this (and pay the "damages")? Since that is unlikely to happen, the school is forced to take action in order to defend itself from a future lawsuit.