Robertson High School was on lockdown for 15 minutes before the end of the school day.
It was the first lockdown at Robertson in at least two years, officials said.
Police said they received a report that two people were in a scuffle at a Robertson High School parking lot and that one broke the other's window.
Someone heard that noise and thought it was a gunshot, so that person called the school. Principal Eddie King then ordered all students into the building and put the campus on lockdown.
He walked down Fifth Street in front of the school and issued orders to teachers via cell phone. The lockdown also applied to neighboring Paul D. Henry Elementary School.
"Don't let anyone on campus," he told one school employee on his phone.
King didn't immediately know the nature of the incident nor did other school employees. The some 500 students in the school followed the order, with the exception of at least one girl sitting against a wall texting. She said she didn't know what was going on.
The lockdown ended right after the end of school after officials found out what had happened. Students promptly started streaming out.
Police Chief Gary Gold said the school wanted to ensure the safety of its students.