A Massachusetts court made the determination after a mental evaluation of the teenager, said Maria Dres, an official in the Essex County Superior Court Clerk's Office.
That evaluation is under seal, and no part of it will be released, Dres added.
Now 16, Chism was 14 in October 2013 when he became "visibly upset" after his Danvers High School algebra teacher, Colleen Ritzer
, spoke about his home state of Tennessee after class, another student said, according to a police affidavit.
Surveillance video later showed Ritzer enter a second-floor girls restroom followed by Chism, wearing gloves and a red sweatshirt with the hood pulled over his head, about a minute later, according to the affidavit. Ritzer, 24, was then allegedly raped with an object and had her throat slashed.
A handwritten note found next to her body said, "I hate you all."
When spotted the next day by a police officer, Chism had a knife, a bloodstained box cutter, a pair of women's underwear, plus Ritzer's credit cards and driver's license, the affidavit said.
Ritzer's body was found in a wooded area near the school, which is 25 miles north of Boston, covered with leaves and debris in an apparent attempt to conceal it, the court papers said. Her throat was slit.
Chism has pleaded not guilty to murder, aggravated rape and robbery charges. He is being tried as an adult.