Christopher Franko pleaded guilty in Suffolk County Court on New York's Long Island on Monday and was immediately sentenced.
Franko, who is 18, was charged with conspiring to attack former classmates at Connetquot High School in Bohemia. He was arrested in 2007 for plotting a similar attack but was deemed a youthful offender. The Family Court file on that case is sealed.
A 2010 indictment on the second plot says Franko made a diagram of the high school, searched online to learn how and where to purchase gunpowder, and how to make a pipe and chlorine bomb.