Yes, those who go to college and make a better salary have an obligation to give back, because the rich should always help the poor. That is not to say that we should be socialist, but it is important to be giving and generous with your money. You should make all you can, save all you can, and give all you can.
Of course they shouldn't have an obligation to pay back. They worked hard for the money they earned, and other people could be getting/doing the same thing if they put in the effort. For those of us who decide to strive for the best, we should be treated equally with those who are lazy, and expected to 'give back' just as much as those who don't work or try to succeed in life at all.