The 19-year-old, who cannot be named, faces a number of charges including conspire to murder over the alleged plot, which was uncovered by police in 2017.
It is alleged he and a 16-year-old co-accused planned to use weapons including bombs, guns and knives to kill as many adults and children as possible.
During the hearing, prosecutor Jim Pearce QC said statements taken from various witnesses - none of whom can be named - demonstrated the man's intent to follow through with the plot.
One witness recalled the man about once a week articulating his plans "to shoot up the school", while another told police the man had spoken of plans to put a bomb at each entrance.
Referring to a statement taken from the school's principal, Pearce said staff raise concerns about the incident most days.
"He speaks about the high level of anxiety felt among the staff and how the school received hundreds of calls at the relevant time from parents," Mr Pearce said.
"There remains continued anxiety for staff and parents around court appearances, particularly around the potential for (the co-accused's) release."
Stephen Ey, appearing for the teenager, dismissed the plot as "fantasies" and said his client had been speaking of his plan for years but had never taken action.
Refusing bail in Adelaide Magistrates Court on Tuesday, magistrate Greg Fisher said the allegations were serious and allowing the man bail would cause concern to his alleged victims.
The younger of the two accused had already been refused bail.
Outside court, the man's mother defended her son and denied he had been collecting guns.
"It's been hell, absolutely pure hell," she told reporters.
The man was remanded in custody to face court again in July.