Justin Carter, 18, made the comment on Facebook during an argument with a friend about a video game he had been playing.
He wrote, 'I'm going to go shoot up a school full of kids and eat their still beating hearts', only two months after the Sandy Hook school shootings, in which 20 children and six adults died.
And although he added 'lol' (laughing out loud) and 'jk' (just kidding) to his post, a woman saw it and reported Justin to police after finding out that he lived near an elementary school in Texas.
The teen was arrested and has been in jail since February. Justin's family are now campaigning for their son's release.
His dad, Jack, said: "These people are serious. They really want my son to go away to jail for a sarcastic comment that he made.
"Justin was the kind of kid who didn't read the newspaper. He didn't watch television. He wasn't aware of current events. These kids, they don't realise what they're doing. They don't understand the implications. They don't understand public space."
His dad said Justin and a friend had been playing a video game called League of Legends.
Mr Carter said: "Someone had said something to the effect of 'Oh you're insane, you're crazy, you're messed up in the head', to which he replied, 'Oh yeah, I'm real messed up in the head, I'm going to go shoot up a school full of kids and eat their still, beating hearts'."
He added that his son had made it clear that the comment was made in jest. But Justin was charged with making a terroristic threat and faces a trial on July 1.
A spokesman for Austin police said that after recent events statements such as the one Justin made are taken seriously.
The teenager's family have set up an online petition - Release Justin Carter - to raise awareness of his case.