Joe Nystrom, who teaches math at a low-income high school here, used to think that only a tiny group of students—the “smart kids”—were capable of advanced coursework, reports the New York Times. But two years ago, spurred by a national program that offered cash incentives and other support for students and teachers, Mr. Nystrom’s school, South High Community School, adopted a come one, come all policy for Advanced Placement courses. Today Mr. Nystrom teaches A.P. statistics to eight times as many students as he used to, and this year 70 percent of them scored high enough to qualify for college credit, compared with 50 percent before. One in four earned the top score possible, far outpacing their counterparts worldwide…