A Heritage High School freshman, 15, has been charged with one count of aggravated battery after punching a sophomore, 16, in a school bathroom on Thursday.
The alleged attacker is being held at a Department of Juvenile Justice detention center.
Investigators have taken the phones of several students who witnessed the attack, Catoosa County Sheriff Gary Sisk said Friday.
Sisk said the victim suffered a concussion and needed 23 stitches around his mouth.
"It is being taken very seriously," the sheriff said.
Sisk said the video shows two boys standing next to each other.
The boys exchanged words, each one accusing the other of bad mouthing him. After about 20 seconds, the bigger of the two boys punched the other boy in the mouth. The other boy kept his hands in his pockets during the altercation.
After the punch, the attacker marches out of the bathroom. The video shows the other boy fall to the ground, seated as someone else holds him.
Heritage High School Principal Ronnie Bradford said the argument seemed to start from a misunderstanding Wednesday. The freshman's girlfriend was pushed in the hallway, and he believed the sophomore shoved her.
But Bradford said other students say this wasn't the case.
"It doesn't appear, based on what I'm hearing from the kids, that he pushed his girlfriend," Bradford said. "Maybe there was some horseplay in the back. Somebody bumped into somebody. They bumped into her."
He said the freshman and sophomore exchanged words and gave each other the middle finger. The boys then agreed to meet in the bathroom on Thursday.
After the punch, friends brought the sophomore to the administrators' office. He then went to a nurse. They originally said the sophomore fell in the bathroom, but they later admitted he was punched. Administrators saw the video soon after.
The boy received stitches, Bradford said. He is not sure whether he has sustained other injuries. The boy was not in school Friday morning.
The freshman, meanwhile, received the school's established punishment for fighting: five days of in-school suspension, the sheriff's investigation notwithstanding.
Contact staff writer Tyler Jett at 423-757-6476 or tjett@timesfreepress.com. Follow him on Twitter @LetsJett.