Philip Bay, 19, entered the pleas to each of the charges in Virginia Beach Circuit Court, The (Norfolk) Virginian-Pilot reported.
Bay was 17 when arrested on April 6, 2009, 14 days before the attack allegedly was to be staged on the 10th anniversary of the Colorado massacre, but is being tried as an adult.
Jury selection began, and defense attorney James Broccoletti said, "The defense is that Philip Bay was legally insane at the time of the acts alleged in the indictment."
Also likely to take the stand are his two younger co-defendants, one of whom is credited with alerting authorities. They both admitted weapons charges last year in juvenile court.
The investigation began with a tip that a Landstown High student had made a threat. An investigator interviewed Bay and led a search of his home.
There, authorities seized 28 explosives, including pipe bombs and Molotov cocktails. They also searched his co-defendants' homes and confiscated a gun scope, computer equipment, journals and notebooks, videos made by the teens and a hit list with students' names.