ST. LOUIS — Starting well before Harvard championed the program with a government innovation award in 2008, officials nationwide have been traveling to the Show-Me State to take notes on the “Missouri model” of juvenile justice. They come to see therapy prioritized over punishment. They come to see where people committed to state custody from juvenile court hold each other accountable while living together in community-based cottages instead of prison settings. And they come to meet staff who believe there’s no such thing as a bad kid — that challenging behavior is an indicator of an unmet nee...