By Matt Petrillo

PHILADELPHIA (CBS) — A grieving mother of an eighth-grade boy who was shot and killed Monday near his North Philadelphia school is demanding answers. She wants to know why her son was able to leave his school before​ its dismissal.

It’s been an emotional day for so many who knew 15-year-old Juan Carlos Robles-Corona. On the ground is a memorial of candles and the teen’s backpack that was left at the scene after the fatal shooting.

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“I need to know why,” said Maria Balbuena, the victim’s mother.

This grieving mother is now planning her son’s funeral after Juan was shot and killed Monday.

“I don’t want no vengeance, I don’t want nobody else to be shot. I just want to know what happened,” Balbuena said.

Dozens of people gathered near the shooting scene at 15th and Susquehanna Streets in North Philadelphia Tuesday to remember Juan’s life.

“When I found out, it really hurt my heart,” Nasim Williams said.

“Don’t nobody deserves to die, but a child?” Beverly Flournoy said.

Police say the eighth-grader at Duckrey Elementary was ambushed about a block from his school and feet from his home. The 20 shots fired alarmed neighbors.

“It was loud, I just didn’t expect it would have been a 15-year-old child so that was very unfortunate,” Monica Kinsey said.

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Juan’s mother says the shooting happened about 15 minutes before her son’s school day ended.

“I hold accountable the school,” she said. “If they would have called me and said, ‘Juan left school,’ I could have called him and said go back to school. He would have went back to school if they would have called me and told me that he left the building.”

A district spokesperson says school leaders are trying to figure out how the teen was able to leave school early.

Back at the vigil, Juan’s classmates wrote messages, saying he’ll be missed.

Now his mother has this emotional plea.

“Please if you’ve seen anything or have any video surveillance of what happened, please — as a mother I just need to know what happened,” she said.

Juan had three younger siblings.

Police say the weapon in the incident has been recovered but no arrests have been made.

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The principal said Juan was “a likable kid who was never in trouble.”