YORK, Pa.—Are you OK? Where are you? Barbara Estep kept texting her daughter, Nylaya Way, who was not responding. Donald Trump had stunned the nation by winning the presidency the night before, and now frightening things were happening at Nylaya’s vocational high school, York County School of Technology. Racial tensions had been building in the school’s corridors, cafeteria, and parking lot throughout the historically divisive campaign. Then, hours after Trump claimed victory in the election, they boiled over as a group of white students held aloft Trump campaign signs and chanted in a hallway, “White power!’’ A brief video clip...