The Obama administration is stepping up pressure on the gatekeepers of billions of dollars in federal aid to crack down on colleges and universities where students fare poorly. Under Secretary of Education Ted Mitchell said he wants accreditors to “do more to address substandard and underperforming institutions” by focusing their efforts on weak schools while reviewing elite schools less rigorously. “Accreditors’ evaluations must increasingly put a premium on student outcomes,” Mr. Mitchell said in a memo laying out the second phase of an executive order announced in November.