A teenage boy accused of plotting a massacre at a South Australian high school will be charged as an adult with solicit to murder.

The teen, who cannot be named, appeared in the Adelaide Youth Court on Wednesday morning on a number of charges in relation to the alleged plot, which was uncovered last year.

He remains in custody after prosecutor Amy Winra opposed bail at an earlier appearance, telling the court the boy and his 18-year-old co-accused plotted to 'kill as many people as possible'.

A teenage boy accused of plotting a massacre at a South Australian high school will be charged as an adult with solicit to murder (stock picture)

She said the pair had attempted to obtain guns and made other weapons which they tested in October, including a Molotov cocktail and what was referred to as 'poor man's napalm'.

They had also made their own body-armour and had machetes and knives to use in the attack, which was allegedly to take place at a high school in the state's Riverland region.

Police believe there would have been significant casualties if the attack had gone ahead before the end of the 2017 school year.

The youth was on Wednesday was committed to Adelaide Magistrates Court on the solicit to murder charge and will next appear at the beginning of June.

SA Major Crimes Superintendent Des Bray previously said this was as close to a U.S. style school shooting as Australia had ever come

At the time of the arrests, Des Bray, SA Major Crimes Superintendent, said this was as close to a U.S. style school shooting as Australia had ever come.

'There is no doubt in my mind that we've prevented a major catastrophe,' Mr Bray said.

Other charges, including using or having possession of a prohibited weapon and unlawful possession and dealing in body armour without written permission were adjourned and will remain in the youth court.

The boy's co-conspirator has also been charged with solicit to murder and is scheduled to face Adelaide Magistrates Court on Wednesday afternoon to seek release on bail.