A 15-year-old Lawrence High student confessed to a witness he "stabbed a kid and cut his head off, killing him," demonstrating with his hand motions, according to a police report released ahead of Mathew Borges' arraignment for the horrific crime today.Borges is accused of killing Lee Viloria-Paulino, 16, last month, chopping off his head and forearms and, the police report states, leaving the torso partially submerged in the Merrimack River.