A GIRL HAS been arrested in the US after she shot three people at her Idaho school before being disarmed by a teacher.

Officials told a news conference yesterday that the two students and a custodian shot are expected to survive their injuries.

Jefferson County Sheriff Steve Anderson said the sixth-grade pupil pulled a handgun from her backpack and fired multiple rounds inside and outside Rigby Middle School in the small city of Rigby, about 150 kilometres south west of Yellowstone National Park.

Sixth-grade students are usually aged 11 or 12. 

A teacher disarmed the girl and held her until law enforcement arrived and took her into custody.

Authorities say they are investigating the motive for the attack and where the girl got the gun.

Dr Michael Lemon, trauma medical director at Eastern Idaho Regional Medical Centre, said the injured adult was treated and released for a bullet wound in an extremity. The bullet went cleanly through the limb, he said.

Both students who were shot are being kept in hospital overnight and one of them might need surgery, Dr Lemon said. One of the students had wounds in two limbs but he said they are both in “fair” condition.

Police were called to the school at around 9.15am local time after students and staff heard gunfire. Multiple law enforcement agencies responded and students were evacuated to a nearby high school to be reunited with their parents.

“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 Iandel Rodriguez said.

“Our teacher went to check it out, and he found blood.”

Iandel’s mother Adela Rodriguez said her daughter and son were OK but “still a little shaky” from the shooting as they left the campus.

“Today we had the worst nightmare a school district could encounter. We had a school shooting here at Rigby Middle School,” Jefferson School District Superintendent Chad Martin said.

“What we know so far is the shooter has been apprehended. There is no further threat to the students.”