Occidental College President Theodore R. Mitchell, a politically connected education expert credited with reinvigorating his Los Angeles campus over the past six years, said Tuesday that he would step down Sept. 1. Mitchell, 49, is leaving the private liberal arts college to become chief executive of NewSchools Venture Fund, a nonprofit group based in San Francisco. NewSchools, founded in 1998 by a group including Silicon Valley venture capitalists John Doerr and Brook Byers, has invested $40 million in efforts to improve public education in urban areas. Mitchell said he decided to take the job at NewSchools because "there is a...