Trouble with manager at work...how do i deal with this?

I am on your boss's side and don't agree with your attitude at all. "May be too tired" is just not a professional excuse to miss something that your boss seems to think is important. While he probably won't fire you for just that, I'm sure he'll be making your life quite miserable until the time you do leave. REAL LIFE is working, and competing, and not whining about exams.