Dana Saltzman, of Commack, pleaded guilty to conspiracy on Wednesday. She is to be sentenced May 4, when she's expected to get 5 years' probation.
The 17-year-old was released without bail. She was not a student at the targeted Connetquot High School last June.
Her lawyer says her boyfriend manipulated Saltzman to participate in the plot.
Christopher Frank pleaded guilty to the same charge last month and was sentenced to 3 to 9 years in prison.
He also admitted in 2007 to an earlier plot to attack the school. He was sentenced to a year in juvenile detention in that case.