Our local CC offers community education classes, not for credit, which are open to anyone. If you take classes for credit, you have to be enrolled. I suppose colleges can set their own criteria for enrolment, but it would be odd for an American student to not have at least a GED to be admitted. Kind of like applying to a PhD program without having a bachelors. There may be exceptions, but you probably would need SAT or ACT results as a minimum. And little Missy Einstein would be sure to have said something about her "geniusy" results if she took those.