Big Hollow Middle School in Ingleside was briefly locked down Thursday after a student found a bullet in a hallway, authorities said.
And later in the day, students were sent home early after a water main broke in the school.
The two incidents were unrelated, Lake County Undersheriff Raymond J. Rose said.
The live, .22-caliber rifle bullet was discovered about 9 a.m. near the eighth-grade classrooms at the school, 26051 W. Nippersink Road, police said.
Sheriff's deputies and a police dog searched the school and student lockers, but no other ammunition or weapons were found.
The lockdown ended about 11:30 a.m. After the search, students and staffers resumed normal daily activities, police said.
That changed about 1:20 p.m. when a water main broke, triggering a fire alarm. Students began to evacuate the school but went back inside when officials realized there wasn't a fire, Rose said.
Even so, officials decided to end the school day early about 2:15 p.m. Normally classes end at 2:45 p.m., he said.
Police are trying to determine where the bullet came from, Rose said. As part of the investigation, they are reviewing video footage from inside the school.
Big Hollow Elementary District 38 officials couldn't be reached for comment Friday.