HILLSBOROUGH COUNTY, Fla.- A 15-year-old Gaither High School student, Larenn Olsson, was arrested on Monday, after admitting to investigators that she threatened to shoot up her school via an Instagram post. The Hillsborough County Sheriff’s Office’s School Threat Assessment and Response Team responded to the school around 10:30 a.m. and took Olsson into custody without incident. Olsson is facing charges of Written or Electronic Threat to Conduct a Mass Shooting or Act of Terrorism, and Unlawful Use of a Two-Way Communications Device. Read: Hillsborough County Sheriff’s K9 Unit Assists In Swif…