Charges upgraded over SA school plot

Two teenagers accused of planning a massacre involving knives and explosives at a South Australian school have had their charges upgraded to conspiracy to commit murder.

The pair, aged 19 and 16, appeared on a video-link in Adelaide Magistrates Court on Friday afternoon, having previously been charged with soliciting to murder and other offences over the alleged plot.

The younger of the two has been denied bail, while an application for the 19-year-old has not yet been decided on.

The court has previously heard the duo encouraged each other to murder staff and students at a high school in the state's Riverland region before the end of 2017.

At a hearing for the older accused last month, prosecutor Tegan Mullan said evidence included conversations between the two about their plan to carry out a mass killing.

"They discuss burning down the school, shooting up the school, blowing things up, making weapons and purchasing firearms," she told the court.

She tendered copies of diaries dated between 2015 and 2017, which she said contained handwritten notes detailing the seriousness of the plans.

Ms Mullan told the court the older teen was "obsessed with school shootings" and had threatened people around the Riverland with descriptions of how the attack would play out.

On Friday, Magistrate Greg Fisher remanded both of the accused in custody to enter their pleas later this month.