The all-clear was given at Lake Brantley High School in Altamonte Springs on Thursday after officials say a science project was reported as a suspicious package.
Seminole County Fire Rescue officials were called to the school, located at 991 Sand Lake Road in Altamonte Springs, after a teacher found the device in a classroom.
The school was evacuated and students and faculty gathered on the football field while crews investigated the incident.
"We were all evacuated and the fire alarm was pulled and we thought it was a standard fire drill. We didn't have much of a clue what was going on," a student said.
Officials said the package turned out to be a student's science project that was accidentally left in the classroom.
Altamonte Springs police described the package as a pack of batteries covered in black tape with wires sticking out of it. The battery pack was intended to power the project, officials said.
Classes resumed after the all-clear was given by authorities.
Faculty later posted a statement on the school's website, explaining what happened.
"A job well done by all faculty, staff and students for an orderly evacuation," the statement said.
No injuries were reported.
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