I'm not sure I have the answer for you, because I'm not sure if grades are actually based on skill level. I thought they were based more on whether your sim has done all the necessary work and gone to class, rather than skill level as such. It seems that as sims go to class, and do homework and write term papers etc, they gain the relevant skills at the same time, which seems to be enough to get high grades. I haven't tried a sim with no skill, but I just had my sim do computer science, and he passed with an A+ even though he started with low programming skill. But the coursework and classes etc improved that skill.. I think this long way of answering is - I think it's more about the homework and completing tasks, more than the skill levels that matter (but I could be wrong).