Family of accused teen 'unaware' of school massacre plot

The brother of an 18-year-old accused of plotting a school massacre with a fellow student has said he believes there's "every possibility" the charges could be downgraded.

The brother spoke to 9NEWS, saying he and the family were "completely unaware" of any problem until the police knocked on the door.

"So for us to have to accept the next day that he's gone from being this normal, bumbly teenager to all of a sudden being charged...does set you back," he said.

The 18-year-old faces charges of solicit to murder, along with four aggravated accounts of threatening to kill.

Court documents allege he threatened to kill students four times between June and November this year, and that he was carrying a weapon on the day of his arrest.

Police have alleged the 18-year-old and a 16-year-old planned to carry out an attack with guns and explosives on a South Australian school in the Riverlands area.

The 16-year-old has also been arrested and charged with solicit to murder. He will face youth court later this month.

Solicit to murder charges carry a maximum penalty of life imprisonment.

The 16-year-old suspect was arrested on November 7 after police received information he and another teenager were planning to attack staff and students at the school.

A friend, who referred to him as a "good kid" who had recently changed, said disturbing messages had been received by other classmates.

Police said the family of at least one of the victims was being supported by the tight-knit community.