Question: I applied to college with no grade lower than an A-. But now I'm worried that I may get a “C" in AP Calculus. Will my acceptances be rescinded, if I do? (All my other grades are A's.)
Colleges will not rescind for a single “C" (although a “D" can be a different story). Students who are REALLY in trouble (i.e., those with multiple C's or worse) should write to the colleges that admitted them (or to just the one they plan to attend) to “explain" atypically low grades. Although, in your case, this isn't necessary, it may make you feel better if you have a valid excuse for the downturn. Such reasons might include an illness that led to getting behind (and lost) in class, a death in the family, other stress at home, etc. However, colleges don't like to hear that you got busy with your extracurricular activities … even when that's the truth. So if you don't have a “good" reason for your grade drop, there's no need to say anything at all.
You also can check out one of College Confidential's most popular discussions, “When Do College Rescind Acceptances?" so you can see that many of your peers are on far shakier ground than you are. If you read through the posts on this long thread, you will get stronger sense of when the college folks are likely to revoke an offer of admission. Go to: http://talk.collegeconfidential.com/ask-dean-topics/903319-when-do-colleges-rescind-acceptances-p1.html
Meanwhile, it's not too late to pull up that Calc grade or, at the very least, to make sure that it doesn't dip any lower. If it does, you really could be in hot water.