Students from the middle school and next door elementary school were evacuated to the nearby high school, and classes were canceled.
The shooting happened on the school's campus, but outside the school building itself, according to police.
Spokeswoman Angela Rambo of Renown Regional Medical Center says two boys are in critical condition.
Gov. Brian Sandoval issued a statement after hearing about the shooting.
"I was deeply saddened to learn of the horrific shooting at Sparks Middle School this morning. My administration is receiving regular updates and the Nevada Highway Patrol is assisting at the scene," he said. Sandoval extended his thoughts and prayers to those affected.
Sparks, a city of roughly 90,000 that sprung out of the railway industry, lies just east of Reno.