The boy, 16, a special needs student at Warren Central High School, wrote on his Facebook page that he would "shoot up the school" after Martin Luther King Day, the Indianapolis Star reported Wednesday.
"Your dreams will be broken by Warren Central, no more Nice Guy," and "I mean what I said and you are going to die tomorrow, every last one of you," were two of the boy's posts.
"My twin daughters came home, they were crying, they said all the kids know about it, and everyone is frightened. We talked about staying home from school Tuesday," said parent Kelly Eubanks.
By Monday evening, many of the school's students and their parents had heard of the cyberthreat, the newspaper said. The school's principal, Rich Shepler, was called at home and the Indianapolis Metropolitan Police Department was contacted.
The principal and police were at the student's house by the end of the night, and he was arrested Tuesday.
"How quickly the administration acted on this shows we take allegations of threats seriously and then move forward to investigate them, and then make sure law enforcement is involved," said a school spokesman, Dennis Jarrett.
The Marion County prosecutor said he wouldn't file felony charges because the threat doesn't fall within the requirements of the state felony intimidation law, but he is checking on what other charges may be pressed.
The boy's parents picked up their son from the Juvenile Detention Center Tuesday afternoon.