WILMINGTON, Del. -- A Delaware judge on Thursday convicted a 17-year-old girl charged with homicide after a school bathroom fight that left a 16-year-old girl dead.

Family Court Judge Robert Coonin delivered his ruling after hearing a week of testimony in the nonjury trial.

An autopsy found that Amy Joyner-Francis died of sudden cardiac death, aggravated by physical and emotional stress from the April 2016 fight at Howard High School of Technology in Wilmington.

Get Breaking News Delivered to Your Inbox

Her assailant was charged with - and ultimately convicted of - criminally negligent homicide for her role in the fight, which was captured on cellphone video.

Two other 17-year-olds were charged with misdemeanor conspiracy. All three, who were 16 when the fight happened, were tried as juveniles. One was found guilty of conspiracy for helping to plan and carry out the attack, reports CBS Philly. A judge rules there was evidence that she kicked Joyner-Francis during the fight.

A third girl was found not guilty. All three girls charged in the case opted not to testify.

Defense attorneys argued that the death of Joyner-Francis, who had a rare heart condition undetected by her doctors, was unforeseeable. They also suggested that she was a willing participant in a fight between “mutual combatants.” Prosecutors argued Joyner-Francis was not looking for a fight but trying to avoid one, and that she would not have died had she not been assaulted.

“Distress, the unexpected nature of the attack, the brute ferocity of it raining upon her, all led to Amy’s death,” deputy attorney general Sean Lugg said during closing arguments this week, adding that the alleged assailant showed “a level of barbarism that reasonably would result in the outcome.”

An image taken from a cellphone video shows Amy Joyner-Francis, right, being punched during a fight shortly before she died. CBS News

It took Coonin about 20 minutes to read his verdict, the station reports.

“While it may be true that Amy Joyner-Francis, due to her condition, would have died from a multitude of stressors, until such an event occurred, if at all, she had a right to live one more day, one more week, one more month or year, until her time, without a contributing cause of another,” Coonin said.

The victim’s family sat silent in the courtroom. But afterward they said they’re 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 Sherry Dorsey, a spokeswoman for the victim’s family.

Attorney John Deckers, representing the girl charged with homicide, argued that she shouldn’t be held culpable for Joyner-Francis’ death because a reasonable person would expect the consequence of a school fight “is not death, but rather discipline.”

“From the very moment this occurred there has been a great deal of confusion as to how this would happen,” Deckers told reporters. “I told her she’ll be fine. She has a family. She’s a good child. This is the first fight she has ever gotten in to.”

Deckers added: “There’s been a lot of children affected by this tragedy. There’s no one who emerges from this unaffected.”

All three girls charged in the case opted not to testify.

The Associated Press is not naming them because they are minors.

Coonin ruled last year that the girl charged with homicide would be tried as a juvenile. Had she been tried and convicted as an adult, the girl would have faced up to eight years in prison. Being declared delinquent, she would be subject to supervision until age 19.

Dr. Richard Ringel, a pediatric cardiologist and professor at Johns Hopkins University, testified that Joyner-Francis suffered from Eisenmenger syndrome, an extremely rare condition for someone her age in which a heart defect combines with severe pulmonary hypertension.

Amy Joyner-Francis CBS News

While Ringel did not dispute the autopsy results, he said there was no way of knowing that Joyner-Francis was at risk of sudden death, any more than a person could predict that an athlete who appears healthy and fit one day suddenly collapses and dies on the playing field the next day.

Prosecutors said that in an online group chat the day before the attack, Joyner-Francis offered advice to one of her friends about a problem involving a boy, telling her friend to “just be careful.” A detective testified that the defendants were later brought into the chat, and that the alleged attacker thought Joyner-Francis - who had warned that someone might betray another person - was talking about her.

A Snapchat posting by one of the defendants that same day shows Joyner-Francis talking to her alleged assailant in the bathroom, purportedly to try to defuse the situation. The posting notes that the girl later charged with homicide was “bouta fight her,” followed by several emojis indicating that a person was laughing so hard she was crying.