BOSTON — For the past two decades, the College Board has moved aggressively to expand the number of high school students taking Advanced Placement courses and tests — in part by pitching the program to low-income students and the schools that serve them. It is a matter of equity, they argue. “What if the best stuff in education were not just for the best to distinguish themselves — but could engage a much broader set of kids?” asked David Coleman, the College Board’s CEO, in a January podcast in