As someone who excelled in school, the most difficult thing I had to learn in the working world was that actual job performance only plays a miniscule part in career success. It's really far more about office politics and your relationships with your coworkers and supervisors/managers.

This can be tough to swallow, especially when you see incompetent idiots who are wildly successful and make way more money than you just because they're part of the good old boys club that goes out drinking and smoking cigars every night.