T.J. Lane, 18, was given three life sentences without the possibility for parole Tuesday for the shooting deaths of the students at Chardon High Schol, east of Cleveland, in February 2012.
Investigators have said he admitted to the shooting but said he didn't know why he did it. Before the case went to adult court last year, a juvenile court judge ruled that Lane was mentally competent to stand trial despite evidence he suffers from hallucinations, psychosis and fantasies.
Lane arrived at the courthouse wearing a blue button-down shirt, but after he sat down, he took the shirt off to reveal the shirt with "killer" written in black marker. When he was given the opportunity to address the court, Lane gave a short, crude statement.
"The hand that pulls the trigger that killed your sons now masturbates to the memory," he said to the victims' relatives.
He ended with "f*** all of you," and then gave the courtroom a middle finger.
Prosecutors say Lane took a .22-caliber pistol and a knife to the school and fired 10 shots at a group of students in the cafeteria. Daniel Parmertor and Demetrius Hewlin, both 16, and Russell King Jr., 17, were killed.
Dina Parmertor, mother of victim Daniel, called Lane "a pathetic excuse for a human being" and wished upon him "an extremely, slow torturous death."
"I hope you have a cold, rough, unkind prison life with monsters like yourself," she said to Lane. "You're a weak pathetic, vile coward."
Lane was not eligible for the death penalty because he was 17 at the time of the shootings.
The Associated Press contributed to this report.