As a college instructor, I hear it every single semester.
"How could you give me a C? I worked really hard in this class!"
And in those 14 words lie two very important lessons.
First, as a teacher, I don't give anyone a grade. The only thing I give is a syllabus, which explains the rules everyone has to play by during the class, including me. After that, the class is kind of on auto pilot; you turn in assignments and earn points based on how well those assignments demonstrate the ideas discussed in class. Later, I add up the points and assign a letter grade based on what the syllabus says your points are worth. That is the grade you get, because that is the grade you've earned.
Nowhere in the syllabus does it state, suggest, or even remotely imply I give you points; it also doesn't say anything about earning points for whining, sighing, or telling me you're an A student. You don't decide if you're an A student; the syllabus does. If you've done the studying, reading, discussing, and writing of rough drafts that helps you learn, you will earn a good grade. Coming to my office to discuss your progress might help as well; coming to my office hours to tell a good joke won't help at all.
Our partnership as teacher and student is meant to enhance your learning, but it cannot replace your learning -- I could easily respect and like you as a person, while your demonstrated knowledge of the material earns you a C. Grades aren't about being friends, sharing the same interests, or filing other students' papers for extra credit -- they are about what you know. If you demonstrate the knowledge, you earn the grade...
...which leads to the second lesson. Some of my students have hired baby sitters and rearranged work hours -- not so they could take my class, but so they could study for it. Free of other obligations, they highlight text, do online research, submit rough drafts of papers, form study groups, come to class an hour early...
...and earn a C.
In the very same class, some students come to class without a notebook, spend a remarkable amount of class time looking out the window, and turn in papers 30 seconds before they're due--papers that could be printed in scholarly journals. These are A students.
Since the C students worked harder, you may think this isn't fair -- but think about your auto mechanic. Would you rather go to the mechanic who opens the hood and gets you back on the road in five minutes, or the mechanic who does an hour of analysis before telling you they know what's wrong, but they don't know how to fix it?
Now that you've answered that question, tell me -- which one is working harder?
Different people learn different things in different ways, and good teachers help every student understand their different mix for success. Your eight hours of studying may instill a stronger work ethic than another student's 20 minutes of studying, but long hours alone won't fix the car if you can't pinpoint the problem -- and the purpose of taking an auto repair class is to learn to fix the car.
If you take the time you need to study and see the big picture, you've achieved the goal. If you've put in a lot of time and can't see a thing, it's time to do something more than hope your teacher will confuse diligence with achievement -- it's time to hit the books until they don't hit back.