A 9 mm bullet was found on the floor in a classroom at Lakeview High School in Litchfield, says Jeffery Villar, the superintendent of Regional School District 20.
The school was placed under a shelter-in-place order, and state police were called to assist.
As part of a safety protocol, troopers and K-9s conducted a full search of the building and did not find a gun.
Villar says a student was the source of the bullet.
He also says the school will take disciplinary action. Police will determine if an arrest will be made.