I think most schools will try to help you out if you have problems dealing with classes. If they agree to get you in, I believe they'll do whatever in their power to keep you in the program. 17-20 hours a semester initially sounds pretty rough to me too. But then most people say that all those classes will address things that are interesting to you and thus will stimulate you to study hard for them. It's not like your undergrad where you have to force yourself to study something like gov or hist that you don't like, for example.
