The girl also was found guilty of third-degree criminal conspiracy in the death of Amy Joyner-Francis, 16.
Two other teenage girls were charged with third-degree criminal conspiracy; one was found guilty at the bench trial in Delaware Family Court. The other was acquitted.
The April 2016 incident was captured on cellphone.
State Prosecutor and Deputy Attorney General Sean P. Lugg had requested that the girl charged with criminally negligent homicide be tried as an adult, but the judge ruled against that motion. The teen was 16 at the time of the incident.
"Today's decision is another reminder for children and adults that their actions have consequences," the Delaware Department of Justice told CNN in a statement. "Hopefully, today's outcome will begin the all-important healing process, and allow a sense of normalcy to return to the school."
No one emerges 'unaffected'
During the trial, the defense argued Amy was a willing participant in the bathroom fight and her death could not be predicted, CNN affiliate KYW
John Deckers, a defense attorney for the teenager convicted of the criminally negligent homicide, said there had been "a great deal of confusion as to how this would happen," KYW reported.
"I told her she'll be fine. She has a family," he said of his client. "She's a good child. This is the first fight she has ever gotten in to."
He added: "There's been a lot of children affected by this tragedy. There's no one who emerges from this unaffected."
Amy was attacked in the bathroom of Howard High School of Technology
in Wilmington on April 21, 2016. The physical and mental toll caused by the attack triggered a preexisting heart condition and caused her to die from a sudden cardiac death, the medical examiner's office said.
"There was an altercation that initially started between two people, and my understanding is that additional individuals joined in against the one person," Gary Fullman, chief of staff to the Wilmington mayor, said then.
The three Howard High students were indefinitely suspended. CNN is not identifying the girls because they are minors.
'You cannot brutalize someone, pummel' them
Amy was an honor-roll student and manager of the wrestling team, CNN affiliate WPVI
quoted City Councilwoman Sherry Dorsey Walker, a family spokeswoman, as saying.
Amy's family sat silent in the courtroom. Afterward, they said they were thankful for the guilty verdict.
"At the end of the day, you cannot brutalize someone, pummel someone in the bathroom and lead to their death and there's no consequence for that action," said Walker, KYW reported.
Sentencing is set for May 23. Juveniles convicted of a crime in Delaware can face probation, house arrest, incarceration and fines.