I would be very surprised if the sequence of events that lead up to that was a sarcastic remark after being called 'insane' followed by 'lol jk'. Context is everything.
Not only that, I could even go so far as to see the police showing up, the kid saying 'no, really, it was a joke', and then the whole thing ending right there, and I wouldn't cry foul. But prosecuting this kid for making 'terroristic threats' is a monumental failure of the justice system.
EDIT: Policeman friend of mine just quoted the Penal Code 22.07 Terroristic Threat verbage for me:
1. Threatening to commit any offense involving violence to any person or property
2. With intent to
a.) cause a reaction of any type by an official or volunteer agency organizied to deal with emergencies
b.) place any person in fear of imminent serious bodily injury
c.)prevent or interrupt the occupation or use of a building, room, place of assembly, place to which the public has access, place of employment or occupation, aircraft, automobile or other form of conveyance or other public place
In his opinion, the kid's actions did not meet the criteria to cause an arrest, much less a prosecution, under this law due to the lack of evidence of intent.
Well, I don't support prosecuting him by any means, or arrest. I don't mind the police investigating to see if there was any intent to act for real though, but the police can do that investigation without him sitting in a jail cell.
That is certainly one possibility, but it's not the only one. Sometimes people tell you exactly what they will do, and then go do it.
In fact there was a school shooting in Virginia just like that a couple of months ago, where the shooter posted his intention to 4chan first.