McInerney didn't speak on Monday, but his attorney said his client is deeply remorseful for the murder of 15-year-old Larry King.
The victim's family delivered impact-witness statements. They said couldn't forgive their son's killer.
"All I really have to say is that he should have life and not get out in 21 years, that's all," said King's grandmother Sharon Townsend.
In a plea deal reached with Ventura County prosecutors last month, the 17-year-old agreed to plead guilty to second-degree murder, as well as one count each of voluntary manslaughter and use of a firearm.
As part of the deal, McInerney received no credit for his nearly four years of incarceration since he was originally arrested. He has no possibility of parole.
"Due to the circumstances, given that juvenile court was not an option, it's difficult to say that we are satisfied with 21 years but we accept that that was the right way to go," said defense attorney Robyn Branson.
"I don't know if I would call it justice, I don't know," said a juror, going by the name Karen. "You know there is people who deliberately go out, stalk people, mutilate them, torture them. I don't see all of them getting 21 years."
A mistrial was declared in September when jurors couldn't reach a unanimous decision on the degree of guilt. McInerney could have faced life in prison if he were convicted of first-degree murder as an adult.
McInerney had just turned 14 years old when he shot and killed King at E.O. Green Junior High School in Oxnard in February 2008.
He will be nearly 39 years old when released from prison.
The Associated Press contributed to this story.