The name on the Instagram post was traced back to the 14-year-old, the television station reported.
When police went to the teen's home, the boy admitted to making the post but said it was a joke, WPBF reported.
The boy was arrested and charged with written threats to commit a mass shooting, according to the television station.
“Threatening to commit a mass shooting, whether it be on social media or through any form of communication, is not a joke,” a spokeswoman for the Boynton Beach Police Department said in a statement. “The Boynton Beach Police Department will vigorously investigate any claim of harm against our community, and arrest any individual responsible. We urge parents to have conversations with their children about the severity of this.”