There shouldn't be any income requirement, for many reasons. The most important of which is that if your parents make a shit ton of money but wont support you going to college for whatever reason, you're totally fucked.

There are other reasons, but that one alone should be more than enough. I also know there are other solutions to that problem, but they're far from simple, and the amount of money it would cost to pay for tuition of the tiny number of families that make that much and wont send their children to a private school is insignificant and would probably cost less than the bureaucracy needed to actually enforce the financial requirements.