Not sure how it works in the US, but here in Canada...it's really hard to get student loans/grants if your parents make a decent income. The govt. basically says your parents should have been saving since you were born to be able to afford to help you out...there's a formula based on your parents' income when determining eligibility for student loans/grants.
So, if it's the same in the States, I think it's wrong to charge rent while your kids are in school. All you're going to do is saddle them with a crushing debt, or cause them to work so much that they don't do as well at school...risking dropout.
And here's a little more food for though: sure, you don't OWE your kids assistance while they are in school technically. You can say, ok...you're done high school..you're on your own now. See ya! But rembember that you are going to get very old, and probably sick someday. When that time comes, and you did the bare minimum for your children (getting them to age 17, as you were required to by law but no more)...how much do you think they're going to want to inconvenience themselves to help you in your time of need? Think on that for awhile.