About 70,000 kids sit behind bars in the U.S. at any given time, according to the Campaign for Youth Justice.

And Richard Ross is telling their stories.

The photographer and University of California — Santa Barbara professor visited about 300 facilities across the country and spoke with more than 1,000 juvenile inmates about how they got to where they are.

"They're all teenagers and teenagers by definition make bad decisions," Ross told Business Insider.

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Most of the young inmates filling America's detention facilities are there for truancy, marijuana, or a whole host of arguably minor crimes.

To fight the growing tide of incarcerated teenagers, Ross said it's time to do away with zero tolerance, a policy he says just doesn't work.

Instead of spending so much time and money locking kids up, he advocates for putting that money back in the community and using it to fund more teachers and effective after-school programs.

Ross, who has published a book on his work with juvenile inmates, said he wants to use his pictures to put a face on the issue of kids as young as 7 being locked up.

"They exist," he said. "They're real people."

Ross will soon head to Miami to visit a teen who has sat behind bars for four years and four months without a trial.

"He has alleged crimes stacked against him but he's not the only kid in this situation," Ross said.

