The ruling came in a nonjury trial for three girls charged in the death of Amy Joyner-Francis. All three were 16 when the fight happened last year and were tried as juveniles. The girl convicted of homicide also was found guilty of misdemeanor conspiracy.
One of the other girls, who in an online post said "we gonna get her.... she's scared," and kicked Joyner-Francis while she was on the floor struggling to defend herself, also was convicted of conspiracy.
Family Court Judge Robert Coonin acquitted the third girl of conspiracy, saying there was no evidence she threatened Joyner-Francis and was even seen on a cellphone video pulling the girl convicted of homicide off Joyner-Francis.
All three girls opted not to testify. The Associated Press is not naming them because they are minors.
The families of Joyner-Francis and her assailant left the courthouse without commenting.
Sentencing for the two girls declared delinquent is set for May 23.
An autopsy found that Joyner-Francis, who had a rare heart condition undetected by her doctors, died of sudden cardiac death, aggravated by physical and emotional stress from the April 2016 fight at Howard High School of Technology in Wilmington.
"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," said Coonin, agreeing with prosecutors that the fatal encounter was an "attack," not a fight.
Coonin said the girl convicted of homicide "struck the first blow without warning, and carried on the relatively brief but violent attack."
Dr. Richard Ringel, a pediatric cardiologist and professor at Johns Hopkins University, testified that Joyner-Francis had Eisenmenger syndrome, an extremely rare condition for someone her age in which a heart defect combines with severe pulmonary hypertension. He said there was no way of knowing Joyner-Francis was at risk of sudden death, any more than an apparently healthy athlete who exerts himself one day then suddenly collapses and dies the next day.
Defense attorneys argued that the death of Joyner-Francis was unforeseeable and suggested that she was a willing participant in a fight. Prosecutors said Joyner-Francis was not looking for a fight, but trying to avoid one.
"I hope that the result today begins the healing for the community, for Howard High School," said State Prosecutor Sean Lugg.
Attorney John Deckers, representing the girl convicted of homicide, argued that his client shouldn't be held culpable because a reasonable person would expect that the consequence of a school fight "is not death, but rather discipline."
However, the judge said the natural and probable consequence of attacking another person is physical trauma, up to and including death. He also ruled that, even considering the assailant's age, her failure to perceive the risk of death from the attack amounted to criminal negligence because it was a "gross deviation" from the standard of conduct of a reasonable person.
Had she been convicted as an adult, the assailant would have faced up to eight years in prison. Being declared delinquent, she could be subject to supervision, and possible incarceration, until age 19. The judge told her she is prohibited from possessing a deadly weapon until age 25.
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." The defendants were later brought into the chat, and the attacker thought Joyner-Francis - who had warned that someone might betray another person - was talking about her.
A Snapchat posting that same day from the girl convicted of conspiracy shows Joyner-Francis talking to her assailant in the bathroom, purportedly to try to defuse the situation. The posting notes that the assailant was "bouta fight her," followed by several emojis indicating that a person was laughing so hard she was crying.
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