Ethan, now 16, is due in court on Monday.
“We can confirm that the shooter is expected to plead guilty to all 24 charges, including terrorism, and the prosecutor has notified the victims,” said David Williams, chief assistant prosecutor in Oakland County.
He was 15 years old when the shooting occurred last year.
His parents had been summoned to school on the day of the shooting to discuss the teenager's writings and drawings showing violence. One of his teachers had found a drawing with a gun pointing at the words: “The thoughts won’t stop. Help me”. There was also an image of a bullet with the message: “Blood everywhere.”
His parents, James and Jennifer Crumbley, declined to take him home but were told to take him to counselling within 48 hours, investigators said.
Ethan was charged as an adult with one count of terrorism causing death, four counts of first-degree murder, seven counts of attempted murder and 12 counts relating to gun use.
The teenager's parents are facing separate involuntary manslaughter charges. They are also accused of neglecting their child's mental healthcare needs and for making a gun accessible to him.
The Associated Press contributed to this report