A former high school in Johnson County — one of the only two-story country schoolhouses left in the state — was named as one of the most endangered historically significant properties in Iowa.The former Sharon High School was one of nine properties added to Preservation Iowa’s Most Endangered Properties list this year, an annual designation of buildings and sites that are “gradually slipping away.” The building is in Sharon Center, an unincorporated Johnson County community less than 10 miles southwest of Iowa City.