Some of the nation’s most prestigious colleges have been enrolling record numbers of low-income students as campuses prioritize economic diversity in the wake of a ban on affirmative action. Students from lower economic rungs remain in the minority on elite campuses, but some schools have accelerated efforts to reach a wider swath of the country. College officials have been steering more financial aid to students whose families are not among the highest earners and seeking out students in poorer areas. Several elite colleges have new freshman classes with more low-income students than ever, including Princeton, Johns Hopkins and MIT.