The three were shot in their extremities and were expected to survive, officials said at a news conference.

Jefferson County Sheriff Steve Anderson says the sixth-grade girl pulled a handgun from her backpack and fired multiple rounds inside and outside Rigby Middle School in the small city of Rigby.

A female teacher disarmed the girl and held her until law enforcement arrived and took her into custody, authorities said, without giving other details.

Authorities said they were investigating the motive for the attack and where the girl got the gun.

She was from the nearby city of Idaho Falls, Anderson said.

Dr Michael Lemon, trauma medical director at Eastern Idaho Regional Medical Center, 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 were being held at the hospital overnight, and one of them might need surgery, Lemon said.

Still, both students were in fair condition. One of the students had wounds in two limbs and might have been shot twice, he said.

Schools would be closed across the district to give students time to be with their families, and counsellors would be available, Jefferson School District Superintendent Chad Martin said.

Police tape surrounded the school, which has about 1500 students in sixth through eighth grades, and small evidence markers were placed next to spots of blood on the ground.

Police were called about 9.15am after students and staffers heard gunfire.

Multiple law enforcement agencies responded, and students were moved to a nearby high school to be reunited with their parents.

The attack appears to be Idaho's second school shooting. In 1999, a student at a high school in Notus fired a shotgun several times.

No one was struck by the gunfire, but one student was injured by ricocheting debris.

Australian Associated Press