Juvenile Crime Surges, Reversing Long Decline. ‘It’s Just Kids Killing Kids.’

A 13-year-old boy ran through the Bronx streets one May afternoon last year, chased by two teens on a scooter. Surveillance video showed him frantically trying to open the doors of an assisted-living facility. The scooter peeled onto the sidewalk and sped toward him. A 15-year-old boy riding on the back pointed a handgun and fired multiple times, police say.

Nearby, 11-year-old Kyhara Tay stood outside a beauty salon after school, eating chicken wings and waiting for her friends to finish getting their nails done. A stray bullet struck the...