The victims all had gunshot wounds to their arms and legs and are expected to survive their injuries, officials said at an afternoon press conference.
The student pulled a handgun from her book bag and repeatedly fired in and out of Rigby Middle School around 9:15 a.m., according to Jefferson County Sheriff Steve Anderson.
The girl was disarmed by the teacher, who immobilized her until police arrived, officials said, without giving further details.
The unidentified sixth grader was from nearby Idaho Falls, police said.
The students were evacuated to a nearby high school. About 1,500 children are enrolled in college, 95 miles southwest of Yellowstone National Park.
“My classmate and I were just in class with our teacher – we were doing some work – and then all of a sudden there was a loud noise, then there were two more loud noises. Then there were screams, ”said 12-year-old Yandel Rodriguez. “Our teacher went to check, and he found blood.
Police are investigating where the girl got the gun and what motivated her to turn it on her young classmates, authorities said.
“Today we had the worst nightmare a school district could encounter. We had a school shooting here at Rigby Middle School, ”Jefferson School District Principal Chad Martin said.
The incident marked the second school shooting in the history of the small state, and the first since 1999.
In 1989, a Rigby student held a teenage boy hostage at gunpoint, but police defused the situation before anyone was hurt.
With AP wires