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.
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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.