Police say a Nevada middle school boy used a semi-automatic handgun to wound two students and kill a math teacher before turning the weapon on himself.

The shooting occurred about 15 minutes before the opening bell at Sparks Middle School on Monday. The student's name wasn't released, and his motive for the shooting wasn't known.

The teacher was being hailed as a hero for trying to protect children from their gun-wielding classmate outside the school. Twenty to 30 horrified students witnessed the shooting as they returned to classes from a weeklong fall break.

Police said between 150 and 200 officers, including some from as far as 100 kilometres away, responded to the shooting.

"In my estimation, he is a hero. ... We do know he was trying to intervene," Reno Deputy Police Chief Tom Robinson said of the teacher who was killed.

Family members identified him as math teacher Michael Landsberry, a 45-year-old military veteran who leaves behind a wife and two stepdaughters

"To hear that he was trying to stop that is not surprising by any means," said his sister-in-law Chanda Landsberry. She added his life could be summed up by his love of family, his students and his country.

On his school website, Michael Landsberry posted a picture of a brown bear and took on a tough-love tone, telling students, "I have one classroom rule and it is very simple: `Thou Shall Not Annoy Mr. L."'

"The kids loved him," Chanda Landsberry said.

Motive unknown

The names of the suspect and two other victims have not yet been released.

"As you can imagine, the best description is chaos," Robinson said. "It's too early to say whether he was targeting people or going on an indiscriminate shooting spree."

Students from the middle school and neighbouring elementary school were evacuated to the nearby high school, and classes were cancelled. The middle school will remain closed for the week.

At the evacuation centre, parents comforted their children.

"We came flying down here to get our kids," said Mike Fiorica, whose nephew attends the school. "... It's really chaotic. You can imagine how parents are feeling. You don't know if your kid's OK."

Sparks, Nev., shooting

Sparks Middle School east of Reno, Nev., was the scene of a shooting early Monday that left two people dead and two boys wounded, police said. (Google Maps)

One of the students injured in the violence that erupted around 7:15 a.m. is out of surgery and the other is doing well, according to police.

The shooting happened on the school's campus and ended outside the school building itself, according to police.

"I was deeply saddened to learn of the horrific shooting at Sparks Middle School this morning," Nevada Gov. Brian Sandoval said in a statement extending his thoughts and prayers to those affected.

About 700 students in 7th and 8th grades are enrolled at the school, located in a working class neighbourhood.

"It's not supposed to happen here," Chanda Landsberry said. "We're just Sparks — little Sparks, Nevada. It's unreal."

The violence erupted nearly a year after a gunman opened fire in Sandy Hook Elementary School in Newtown, Conn., leaving 26 dead. The Dec. 14 shooting ignited debate over how best to protect U.S. schools and whether armed teachers should be part of that equation.

In a statement on the website of Sandy Hook Promise, a gun control advocacy group, Nicole Hockley, whose son Dylan was killed in the shooting said, "It's moments like this that demand that we unite as parents to find common sense solutions that keep our children — all children — safe, and prevent these tragedies from happening again and again."