Kameron Asgari died from his injuries on Tuesday and the girl, who has not been identified, was left in a critical condition in a separate incident nine hours later.
Both children are believed to have found the weapons at their homes.
Kameron Asgari was in the fifth grade at Helen Jydstrup Elementary School in Las Vegas.
It was unclear if the boy attended school the morning he shot himself, said David Roddy, spokesman for the Clark County School District.
Privacy laws prohibited school officials from disclosing whether Kameron Asgari had been involved in any disciplinary actions or requested counseling before the shooting.
Both Las Vegas incidents are under investigation. The medical examiner has not released an exact cause of death for Kameron Asgari.
Counselors were supporting his classmates and teachers.
In a letter, Principal David Fydman said: “Please monitor any signs of grief or behavioral changes in your child/children as this loss may affect them in unexpected ways.
“It is important to be honest with him or her and to allow them to express feelings of grief, anger and/or disbelief.
“Reassure your child that there is always someone with whom he/she can talk and that the many mixed emotions they might be feeling, and may feel for some time, are normal.”
Bill Cassell said the shootings should remind gun owners to securely store their weapons and to instruct children to avoid touching any gun found inside the home.
“When we have the awesome power of a firearm, we have to control it and protect everyone from an unwanted occurrence,” he said.
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