But the kids wasn't arrested--he was expelled. Similarly, King would be very likely to be dropped by his publisher if he DID write about blowing up his neighbors house using his and their names.
It happens all the time in the adult world too that while you are not criminally liable if you express hurtful opinions in a public way, you may well find yourself out of a job (which is the closest adult equivalent to school I guess). There was an NPR reporter fired this week for expressing his opinion publicly (on, I think, the Bill Reilly show) -and that was not even a threat like these are.
Not being one to listen to much rap I did not know there is a lot of music out there making specific violent (and death) threats against specific living people the rapper has access to on a regular basis. The songs we do have are pretty violent and use pretty terrible language but are not specific like that (that i have noticed). I actually do have an issue with that as well. I think their labels would be well with in their rights to drop them for this (which obviously they do not want to do if they are putting the music out there

).