Page County High School students were sent home as police searched the building for a bomb.
Officials said the threat was discovered shortly after classes began Thursday morning, and the school's 730 students were evacuated.
Police have found no explosive device at the school in Shenandoah, but a comprehensive search has been delayed until a bomb-sniffing dog can be obtained.
Thursday's threat was at least the fourth in the last eight months. A 16-year-old girl pleaded guilty to charges in an October 31 threat, and a 15-year-old girl pleaded guilty in a November 13 case.