Two teenagers charged over an alleged plot to carry out a massacre at a Riverland school last year will stand trial after pleading not guilty to conspiring to murder staff and students.

The teenagers, now aged 19 and 17, were arrested in November over their alleged plan to attack the school with guns and explosives before the end of term last year.

Today they appeared in the Adelaide Magistrates Court by video link from custody and they both pleaded not guilty to conspiring to murder students and staff between June and November 2017.

Magistrate Elizabeth Sheppard ordered the younger teen be tried as an adult.

Last month, lawyers for the two teenagers applied to have the charges thrown out of court, saying their clients were just "talking tough".

The Adelaide Magistrates Court also heard that the two teenagers were "obsessed" with school shootings in the United States, including the infamous Columbine massacre.

Lawyer Greg Mead SC told the court in July that the accused were just "two moody teenagers expressing their dissatisfaction with their lives and the world".

"It was an over-dramatic way of expressing their apparent depressed state of mind," Mr Mead said.

"Is this serious criminal behaviour or is it two youths acting tough?"

But Prosecutor Jim Pearce QC told the court the teenagers had formed a suicide pact and were making molotov cocktails and homemade explosive devices.

He said a conspiracy to murder charge would be made out of if the prosecution could prove there was a pact between the boys to carry out the offence, even if they later withdrew their plans.

They were both remanded in custody to be arraigned in the Supreme Court on August 27 at which time a trial date will be set.