The victim, identified only as Diego in a statement from the Riverside County Sheriff's Department, was pronounced clinically dead Tuesday evening.
MORE: Moreno Valley parents demand answers after middle school assault
"Preparations by Diego's family are underway for organ donation to transform this tragedy into the gift of life for other children," the news release said.
Two other students who were involved in the incident, which was recorded on video by a bystander, remained in custody following their earlier arrests on suspicion of assault, authorities said. Their identities were not disclosed due to their ages.
School officials held a press conference Wednesday.
"Our community and the Moreno Valley Unified School District family has suffered an immeasurable loss," said Martinrex Kedziora, Moreno Valley Unified School District superintendent. "On behalf of the Board of Education, I want to send my deepest condolences to the family and friends of our beloved student."
A remembrance ceremony was held at the middle school, with counselors available to speak with students, teachers and community members.
Heartbreaking words were shared by his friends as hundreds gathered outside the school.
Parents have been demanding answers from school administrators in the wake of the attack.
"Kids are being bullied and they're not doing anything," said parent Karla Palumbo. "We've asked them to change the policies, the rules, a little bit more stricter, but that hasn't happened."
At the vigil, an angry group surrounded the school district superintendent -- some calling him a coward -- as he made his way into the school.
Earlier in the day, Kedziora told reporters extra security has been added and instances of violence on campus isn't taken lightly by the school.
When he was asked if Diego's death could have been prevented, Kedziora said the incident was still under investigation and he couldn't comment on it.
"On behalf of the City Council, I want to extend my sincere condolences to Diego's family and friends. No one should have to endure the pain of such a terrible loss," Moreno Valley Mayor Yxstian Gutierrez said in a statement. "This is a life-changing tragedy for everyone involved, including the staff and students at Landmark Middle School."
Anyone with information about the incident is asked to call sheriff's Investigator Joshua Manjarrez at (951) 955-2777.
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