A Tennessee teenager charged with fatally stabbing the principal of his Christian high school pleaded guilty to second-degree murder on Tuesday. Accepting a plea deal at the Shelby County Justice Center in East Memphis, Eduardo Marmolejo was sentenced to 35 years in prison without parole for the 2011 murder of Suzette York.

According to the Associated Press, Marmolejo, 18, apologized to York’s family and said he had changed. "I apologize for my bad behavior," Marmolejo told Judge W. Mark Ward. "I'm not the same man I used to be. I'm a better man."

"Instead of thinking and trying to make it better, I made it worse," he added. "I'm now in a better state of mind."

Police discovered York, 49, in a classroom at the Seventh-day Adventist school Memphis Junior Academy in August of 2011, lying in a puddle of her own blood with multiple stab wounds to her neck and skull. Marmolejo, who was 16 at the time, told police that he had been planning the murder since the close of the previous academic year. He said he was angry with York and other school administrators for removing privileges and had known ahead of time that she would be alone in her classroom.

Homicide detectives who testified in the juvenile court hearing said Marmolejo planned the killing out months in advance, reading up on combat strategies and even studying how to stab someone without allowing them to scream. Detectives reported that he tried to dispose of the murder weapon, a SWAT folding knife with a 3.5-inch-long, by flushing it down a toilet, and changed out of his clothes after the attack.

In 2011, a juvenile court judge ruled that Marmolejo should be tried as an adult. He was indicted on murder charges in February 2012. Despite doubts raised by his defense attorney Leslie Ballin that he would be able to follow legal proceedings, Marmolejo passed a mental evaluation that reported he was sane and competent to stand trial. A psychologist who testified as a witness before the juvenile court judge said Marmolejo could communicate rationally with his attorneys, but conceded that he still harbored fantasies in which he believed he was a soldier.

Ballin describes Marmolejo as a loner, and says he probably still battles psychological problems. "There are no winners in this case," Ballin said. "It's just a tragedy from A to Z."

Weeks ago, Ballin and prosecutor Reggie Henderson announced that they were cooperating on a plea deal. Henderson said the evidence against Marmolejo “was overwhelming," and believed a trial would likely end in a conviction, but pursued the plea deal at the behest of York’s husband, who wanted to avoid a trial.

"A confession, forensic evidence, it was a very, very strong case," Henderson said.

Marmolejo’s mother, Marcela Rodriguez, told reporters in Spanish outside of the court on Tuesday that she was happy with the plea deal"We are with Jesus, and he has given us the strength to survive this," Rodriguez said.