Triad High School was evacuated Wednesday afternoon while the Madison County Sheriff's Department investigated a "potential threat," according to Capt. Mike Dixon.
Dixon declined to describe the threat.
The Madison County Sheriff's Department and the Troy Police Department searched the school and about 2:30 p.m., the school was declared secure.
The students were evacuated from the school about 1:15 p.m. Wednesday, according to a statement released by the school district. Students remained evacuated near the lower athletic fields until dismissal at 2:20 p.m. and were supervised by staff.
Triad Community School District 2 Superintendent Leigh Lewis declined to comment.
According to the school's website, Wednesday was the last day of regular classes before exams. Final exams will be held as scheduled Thursday. Winter break begins at 2:05 p.m. on Friday.
The school, where about 1,200 students attend, is located at 703 E. Highway 40 in Troy.
Triad was on lockdown a year ago after two .22-caliber bullets were found in a stairway by administrators. The lockdown lasted an hour and a half. Students were kept in their classrooms.
The bullets were found on Dec. 10, 2012. All 1,200 lockers at the school were checked after the discovery. During the search, an additional bullet was discovered.
In March 2007, a Triad student was arrested after live .22-caliber bullets were found on the floor of the school. The school went on lockdown. A student was questioned, but an administrator said that the pupil was a hunter and had forgotten to remove the bullets when he came to school. No charges were filed in that incident.