Pet peeve, but why do people title threads with titles that have nothing to do with the actual topic? This isn't a biology question, but rather whether to withdraw from a class. (Sorry if I'm being too particular, but I think I have a sinus infection, so I'm grumpy at the moment).
But, I did want to provide some insight to your dilemma. Personally, I wouldn't drop your class but try to do the best possible with the exams you have left. If you study really hard, you can still do well. And if you do end up with that B-, well, I have a suggestion for that, too. From the pharmacy school I'm going to, they suggested that a way to make up for a lower grade in a class was not so much to re-take that class (as it seemed like an easy way to turn a B- into an A) but rather to take a harder class in that field and do well in it. This can help make up for a bad day in an exam or a not-so-good teacher.