A North Attleborough High School student accused of threatening to “shoot up the school” was arraigned Monday in Attleboro District Court, according to the Bristol district attorney’s office. Patrick Skrabec, 17, was arrested Friday at the North Attleborough police station, said Gregg Miliote, spokesman for Bristol District Attorney Samuel Sutter. Skrabec entered a plea of not guilty, officials said. Skrabec was charged with making a bomb or hijack threat and is being held pending a dangerousness hearing scheduled for Thursday, he said. The judge will then decide whether to hold Skrabec for up to 90 days while his case is processed, or will set bail, Miliote said.