In theory by accepting and graduating students on merit, who then become successful and donate back to their institutions. Not by using their tax exempt status to solicit beneficial tax deductions from the rich in exchange for accepting their children who otherwise wouldn't be accepted.
It should be illegal for any school that is either: a) tax exempt; or b) a state institution to admit a student in exchange for money.
Mostly donations from graduates who have done well. I suppose if you want to look at the data you could figure out how much of that is some kind of quid-pro-quo to get those people's otherwise not terribly worthy kids in vs how many are simply moved to contribute.