The two wounded boys were taken to a local hospital in critical condition following the shooting at Sparks Middle School in northern Nevada, the Reno Gazette-Journal reported.
The teacher who was killed was identified by his family as Michael Landsberry, a 45-year-old math teacher and military veteran.
He leaves behind a wife and two stepdaughters, his sister-in-law told The Associated Press.
Kyle Nucum, a 13-year-old student at the school, told the Gazette Journal he was outside when the gunfire began.
“I thought it was a firecracker at first, but the student was pointing a gun at the teacher after the teacher told him to put it down, and the student fired a shot at the teacher and the teacher fell and everybody ran away," he said.
Reports of the shooting started coming in at 7:16 a.m. Monday.
The shooting happened on the school's campus, but outside the school building itself, Fox News reported.
The student's motive was unclear.
"As you can imagine, the best description is chaos," Tom Robinson, deputy chief of the Reno Police Department, told CNN. "It's too early to say whether he was targeting specific people or just going on an indiscriminate shooting spree."
Sparks is a city of 90,000 just east of Reno.