The 14-year-old told police he planned to hold his class hostage, Maricopa County Attorney Andrew Thomas said Monday.
When the teen was arrested two weeks ago, county officials said he had plotted a "Columbine-like" scenario, comparing his plans to the 1999 deadly shooting massacre at Colorado's Columbine High School.
The teen also was charged with aggravated assault with a deadly weapon and kidnapping, as well as a misdemeanor count of carrying a weapon on school grounds, Thomas said Monday.
He will be tried as an adult, said Barnett Lotstein, a spokesman for Thomas.
The boy was arrested at his home in Mesa on March 23. Police said they discovered weapons in his backpack, including a .25-caliber handgun loaded with a full clip of bullets and one in the chamber, plus a box of 28 rounds of ammunition, Mesa Police spokeswoman Diana Tapia said.
Police also found three large knives, a lock, a metal chain and two rolls of duct tape in the backpack, Tapia said.
Police said they were alerted by a girl who said the boy confronted her with a knife.
Holly Hosac, another Mesa police spokeswoman, said the teen was a good student but apparently had trouble with another boy at school.