SPARKS, Nev. (AP) — A student at a Nevada middle school opened fire with a semi-automatic handgun on campus just before the starting bell Monday, wounding two 12-year-old boys and killing a math teacher who was trying to protect children from their classmate.

The unidentified shooter killed himself with the gun after a rampage that occurred in front of 20 to 30 horrified students who had just returned to school from a weeklong fall break. Authorities did not provide a motive for the shooting, and it's unknown where the student got the gun.

Teacher Michael Landsberry was being hailed for his actions during the shooting outside Sparks Middle School.

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

Both wounded students were listed in stable condition. One was shot in the shoulder, and the other was hit in the abdomen.

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

Landsberry, 45, was a military veteran and leaves behind a wife and two stepdaughters. Sparks Mayor Geno Martini said Landsberry served two tours in Afghanistan with the Nevada National Guard.

"He proudly served his country and was proudly defending the students at his school," Martini said.

On his school website, 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," his sister-in-law Chanda Landsberry said.

She added his life could be summed up by his love of his family, his students and his country.

"To hear that he was trying to stop that is not surprising by any means," she said.

Police said 150 to 200 officers responded to the shooting, including some from as far as 60 miles away. Students from the middle school and neighboring elementary school were evacuated to the nearby high school, and classes were canceled. The middle school will remain closed for the week.

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"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."

At the evacuation center, parents comforted their children.

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

The shooting happened on the school's campus and ended outside the school building, 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, in a working class neighborhood.

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

The mayor praised the quick response from law officers who arrived at the scene within three minutes of a flood of 911 calls to find the gunman already dead from a self-inflicted gunshot wound.

"They got it under control very quickly and shut down the scene," Martini said.

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

The Washoe County School District held a session in the spring in light of the Newtown tragedy to educate parents on its safety measures. The district has its own 38-officer police department. No officers were on campus at the time of the shooting.

Sparks, a city of roughly 90,000 that sprung out of the railway industry, is just east of Reno.

"You see it on TV all the time. You just don't think it's going to happen to you," Martini said.

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Associated Press writer Michelle Rindels in Las Vegas and news researcher Rhonda Shafner in New York contributed to this report.