According to Jefferson County Sheriff's Office, the child, who has yet to be identified, pulled the loaded gun from her bag shortly after 9 AM and began firing multiple rounds, inside and outside of the school.
Police do not yet have a motive for the attack — nor do they know where she got the firearm.
"We don't have a lot of details at this time of 'why' — that is being investigated," Sheriff Steve Anderson said, per AP. "We're following all leads."
Terrified students described hearing loud bangs, while classes went into lockdown before being quickly evacuated. Others described seeing blood spilled in the hallways.
"Me and my classmate were just in class with our teacher — we were doing work — and then all of a sudden, here was a loud noise and then there were two more loud noises. Then there was screaming,” 12-year-old Yandel Rodriguez told AP. "Our teacher went to check it out, and he found blood."
All three victims were hit in extremities; one of the girls appeared to have been struck twice. Doctors described the conditions of both girls as fair, adding one might require surgery, but that they may be released before the weekend.
A GoFundMe for the janitor Jim Wilson (pictured above, right) described him as a beloved custodian who had worked for years at the school; it has already raised $20k.
Another started by one of the victim's grandmother's said her granddaughter was just 11 years old.
"My beautiful granddaughter Addison was a victim of a school shooting in Rigby Idaho," she wrote. "She was shot and is recovering in the hospital. She is the sweetest most beautiful person. She loves everyone, always takes up for the underdog. She is a wonderful big sister."
"I'm praying that this does not ruin her innocence and the love she has for people. She is the bravest 11 year old girl I know she handled this better then we did. If we were breaking down she would say please don't cry I'm okay. We pray for the other 2 victims and hope recovery for all. We just want Addison, mom and dad to heal from this horrible day."
On a Life in Rigby Facebook page, the hero teacher praised her colleagues, and thanked the community for the outpouring of support.
"My heart is touched by all the incredible outpouring of love I've received. Thank you," she wrote.
"I don't want money, I just appreciate the incredible support of Rigby!!! I love my students so much! It is why I teach!! They make my heart happy every every day!!!!"
"All of the staff at Rigby did their part and kept our wonderful children safe! Thank you! I love you all and we will get through this together."
Jefferson County Prosecutor Mark Taylor said criminal charges against the girl — believed to be around 12 years old — wouldn't be decided upon until the investigation was complete; however he added that three counts of attempted murder were a possibility.