Hallandale High School was placed on lockdown Thursday after a student posted a video on social media showing him holding a loaded magazine, authorities said.
Broward County Public Schools spokeswoman Nadine Drew said the school was under a "code-red" lockdown, the highest security measure that doesn't allow students, teachers or parents in or out of the school.
Students, who were supposed to be released early at 11:30 a.m., were not released until the all-clear was given just before 1:30 p.m.
Hallandale Beach police said officers responded to the school at 720 NW 9th Ave. after the 10th-grader posted the video on Snapchat.
Police said all students inside the gym where a pep rally was being held were searched, including the boy who posted the video.
No ammunition or weapon was found at the school, but authorities believe that the video was taken Thursday at the campus.
The teen is being questioned by police.
It's unclear whether he will face any charges.