If you keep comparing yourself to people you know are better than yourself, you are going to crush your own morale and as a result, not improve very much.
I always tell my students that the key goal is to perform to the full extent of their own capabilities. People can have a bad day and do poorly on an exam, but the trick is to be preparred and ready to take it. Anything beyond that is circumstancial and very dependant on the situation.
My advise to you is to look at your grades and ask yourself "What do I want to improve?" or "What can I improve?". For the next semester or school year, put specific goals and work dilligently to reach those goals. That is far more productive and easier than to set impossible standards and measure yourself against them. :P