A jury has found teen killer Philip Chism guilty of raping, strangling and stabbing his freshman math teacher to death in 2013.

Rejecting his defense team's insanity plea, the jury found the 16-year-old guilty of premediated murder in the first degree with extreme atrocity. He was acquited of one count of aggravated rape but found guilty on a second count of aggravated rape. He was also found guilty of armed robbery for the theft of her credit cards.

Chism remained unemotional, showing no emotion as the verdicts were read.

The jury of eight men and four women had been deliberating since yesterday on the charges of first-degree murder, aggravated rape and armed robbery.

Lowy told the court just before the foreman read the verdicts today, the jurors "will never be the same people for having decided this case." He thanked them for their service.

A pre-sentencing status hearing is scheduled for Dec. 22 at 10:30 a.m. Chism will not be present. He is facing the possibility of three life sentences. Teen offenders are held in Department of Youth Services custody until he is 18, when they are transfered to adult prisons. Under the state's new teen offender law, a life sentence carries the possibility of parole after 15 years.

The Tennessee native’s lawyers had admitted Chism raped and killed Danvers High teacher Colleen Ritzer, 24, on Oct. 22, 2013, in a bathroom at the school where she had taught for less than two years, but argued the then-14-year-old couldn’t be held criminally responsible because he suffered from a long-untreated mental illness — likely early onset schizophrenia, they said.

Salem Superior Court Judge David A. Lowy had told the jury yesterday that Chism could conceivably be committed in a mental-health facility for life, but that is the court’s decision alone.

Prosecutors had countered with several psychological experts who said Chism's inconsistent answers to their tests suggested he was faking mental illness. They also cited his extensive preparation — pulling on gloves while stalking the teacher into the restroom where he attacked her — and his methodical disposal of her body, wheeling it in a recycling bin out of the school and into the woods.

Ritzer, of Andover, was found partially naked and sexually posed in a shallow grave in woods beside the school, buried beneath leaves, pine needles and twigs.

A note handwritten in blue ink that read, “I hate you all,” marked the macabre scene.

Police who testified described finding evidence of the crime scattered around boulders, under bushes and in a stream, including the book bag Ritzer received as a gift and the school recycling barrel Chism used to move the mortally wounded Ritzer from the school.

Jurors heard 53 witnesses in total testify over 13 days. They shared the jury deliberations room with 140 trial exhibits – among them, autopsy photos of Ritzer.

Ritzer’s parents Tom and Peggie, were present for every day of the disturbing three-month trial.

— laurel.sweet@bostonherald.com