I blame the collectivisation of the family and the prosecutorial steerage of it. Parents do not have enough authority to discipline their own children, and too much of the village is doing the raising.
Parents aren't taking responsibility. That's the bottom line. I don't know where it comes from but I see it all the time. Parents are disconnected and worse, lack the fortitude to provide punishment. They're pushovers. Teachers get attacked in classrooms, schools attempt discipline, and parents cry foul. There's an entire generation that's got their heads in the sand here.
I think the approach with the handcuffs was possibly a little excessive but not too far out of line considering the magnitude of the offense. Providing public example for their peers was almost certainly a goal as well.
OTOH, I'd like to see these kids lose most entertainment privleges, and sentenced to a year of community service that will expose them to the plight of the homeless as well as prison conditions endured by their victim. Face to face restitution is a powerful deterrent and teaching aid.