“I haven’t been this nervous since I danced with Tina Turner,” Oprah Winfrey said Tuesday, following her teaching debut at Northwestern University’s Kellogg Graduate School of Management. Winfrey and longtime beau Stedman Graham are teaching “Dynamics of Leadership” at the highly ranked business school, where 110 students snagged a spot in the 10-week course. “I speak to millions every day, but it’s different one-on-one when people have paid to hear you say something meaningful,” Oprah said. After the nervousness subsided, she said she quickly got into the swing of teaching. She gave herself a B.