A jury has acquitted a North Attleborough High School student of threatening to shoot up his school. Patrick Skrabec, who spent six days in jail over the Christmas holiday after his arrest in 2012 when he was 17, was found not guilty Thursday of threatening to commit a crime and disturbing a school assembly after a two-day trial and 45 minutes of jury deliberations. Prosecutors said he made his comments in a math class in December 2012. He said it was a private joke between friends. His lawyers said he had no access to guns, no history of disciplinary problems, and no criminal record.