A 14-year-old boy accused of plotting a Columbine-style attack on a suburban Philadelphia high school admitted in juvenile court Friday that he illegally stockpiled weapons.

Dillon Cossey, 14, will be held in juvenile custody for as long as seven years.

Cossey, a home-schooled student, was arrested earlier this month. He had amassed a stockpile of weapons, including a 9 mm semiautomatic rifle, about 30 air-powered guns modeled to look like higher-powered weapons, swords, knives, a bomb-making book, videos of the 1999 Columbine high school attack in Colorado and violence-filled notebooks, authorities said.