Duncanville police say a bullet was found inside the school cafeteria at the Village Tech Charter School.
A school administrator called 911 just after 1 p.m. Tuesday after finding the single live round of ammunition.
It was found in the school cafeteria during the lunch hour.
Duncanville police responded to the scene and provided security to the school while administrators conducted a search of the facility and students.
On Sunday, two students were taken into police custody after a social media post threatened Duncanville High School. That threat prompted a police investigation and increased security measures when students return to school Monday.