WASHINGTON — Six decades after the Supreme Court outlawed separating students by race, stubborn disparities persist in how the country educates its poor and minority children.A report Tuesday by the nonpartisan Government Accountability Office found deepening segregation of black and Hispanic students at high-poverty K-12 public schools. These schools often offered fewer math, science and college prep classes, while having disproportionally higher rates of students who were held back in ninth grade, suspended or …