They usually start with a harmless-looking social-media post: a call for teenagers to “pull up” and “shake some ass,” as one online flyer put it. These get forwarded across the internet, and soon masses of young people are surging into an urban plaza or park, often terrifying any adult who happens to be nearby. Teen takeovers can be “amazing,” like an outdoor street party, I was told by one 17-year-old named Jaden who said he had attended many of them—“but only until the fighting starts.” The fi