Oprah Winfrey was such a smash with Northwestern University business students last fall that she’s been invited to come back for more. The talk-show queen and her longtime beau Stedman Graham taught “Dynamics of Leadership” at the university’s J.L. Kellogg School of Management, and she has now accepted an offer to teach again this coming fall. Speaking to the campus paper (Northwestern’s in Evanston, Ill., just outside Chicago), Prof. Winfrey said, “I gave myself a B, and I’m coming back to get an A because I now know how to get it.”