Schley, a star running back and community volunteer, thought the gun was empty, according to Philadelphia Daily News.
"It was horseplay that went wrong. One kid trying to impress another," said his mother Michelle Schley.
The shooting happened before he and a friend were to leave to attend a party.
And although Justin was an only child, his mother told the paper that she was "holding up pretty well."
"I'm thanking God for the time He allowed me to have Justin. It was a wonderful experience. And I keep hearing such nice things from the kids who knew him. They all seemed to love him."
Justin Schley was scheduled to begin classes next Monday at Millersville where he would have played football.
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Services will be held on June 30 from 9 a.m. to 10 a.m. at the Enon Tabernacle Baptist Church in West Germantown.