Nearly one in six, or 15 percent, of students that Harvard University offered admission to this spring were African-American. But step onto Harvard’s campus, and the student body looks far less diverse.  In fact, according to the most recent federal data available, just 8 percent of Harvard’s undergraduates are black and just over 9 percent of the incoming freshmen last year were black.  So what happens between admissions and the first day of classes?