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A teenager in Michigan who killed four students with a semi-automatic handgun could spend the rest of his life in prison without the possibility of parole, a judge has ruled.

Ethan Crumbley, 17, pleaded guilty to terrorism and murder charges last year.

He was 15 when he opened fire at Oxford High School, around 30 miles (48km) north of Detroit, in November 2021.

The sentencing is set for December. He faces a minimum term of 25 years in prison.

Automatic life sentences for criminals aged under 18 have been ruled unconstitutional by the US Supreme Court - even for murder - but they can be imposed by a judge based on the seriousness of the crime, the maturity of the child and other factors.

During an online hearing Friday, Judge Kwamé Rowe cited psychological evaluations and said that the attacker's planning, work and social life "does not illustrate the hallmark immaturity of a child".

The judge noted that before the attack the attacker wrote 22 pages in his journal about school shootings and conducted significant research online about other school shootings and potential maximum sentences.

He killed four students and wounded seven others during the rampage.

The shooter pleaded guilty last year to 24 charges, including terrorism, murder, assault and possession of a firearm.

During Friday's hearing, he mostly looked down and did not visibly react when the judge announced his ruling.

His parents, James and Jennifer Crumbley, are awaiting trial on four counts of involuntary manslaughter.

Prosecutors say the Crumbleys bought their son a gun while ignoring clear warning signs that he was about to commit violence. They have pleaded not guilty to the charges.