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CASTLE ROCK, Colo. — They plotted to shoot up their Highlands Ranch high school. They tried to get their hands on guns. And they were just 16 at the time.

On Wednesday, the second of two teenagers accused in that planned attack reached a plea deal with the Douglas County District Attorney’s Office.

Sienna Johnson pleaded guilty to an adult charge and a juvenile charge. They are guilty pleas that will, at the very least, land her in juvenile prison until she’s 21.

At a time most teenagers are focused on driving, their friends and school activities, Johnson’s attention and that of her friend Brooke Higgins turned to killing their classmates and teachers at Mountain Vista High School in December 2015.

“This case was extremely troubling for everyone involved,” District Attorney George Brauchler said.

He had to figure out how to ensure public safety and adequate punishment for the then-16-year-olds.

“My goal is for the public to understand how this happened, why it happened and what are the consequences. This was not a couple of kids that said, ‘What if?’ This was more than that and we’re treating it like more than that,” Brauchler said.

It means Johnson agreed to plead guilty to an adult charge of felony menacing and a juvenile charge of conspiracy to commit first-degree murder with a sentence enhancer of violent juvenile offender.

Higgins, agreed to a similar deal, pleading guilty to conspiracy to commit first-degree murder, an adult charge, and first-degree murder solicitation, a juvenile charge.

But her felony goes away if she stays out of trouble. If she doesn’t, Higgins faces eight to 24 years in adult prison. Johnson will remain a convicted felon no matter what.

“(Johnson’s family was) willing to take a felony conviction to get out from under the potential for eight to 24 years (in prison),” Brauchler said.

It is a plea deal Brauchler said protects the public, but sets up the teens to be rehabilitated.

“At the end of the day, I have great hope these girls will end up growing up to be responsible, law-abiding members of society, that pose a risk to no one. We cannot say that today. That’s why we put this particular sentence in place,” he said.

Johnson will be formally sentenced Aug. 11. She will also be on probation for four years after she’s released.

If she violates that probation, she’ll be sent to adult prison for two to six years.

Higgins received three years in youth corrections and four years probation. But she faces the much harsher sentence if she gets into trouble during probation.