LAKELAND, Fla. — Authorities in Florida say a 16-year-old boy posted selfies online with captions that threatened his high school.

News outlets report the Kathleen High School student was arrested Thursday by Polk County Sheriff’s deputies and charged with writing a threat to kill or injure. Deputies say the teen posted a selfie to Instagram on Wednesday with the caption “Active shooter drill tomorrow lol it would be a shame if it was real.”

That post was reported to authorities, who then interviewed the teen and found a second Instagram post on his cellphone that said, “someone will die today.”

The sheriff’s office says the teen told deputies the posts were a joke “and that he knew it was stupid.” Polk County Sheriff Grady Judd says it wasn’t a joke, but a felony.