To B or to A?

Yesterday I was thinking, there is one girl in my class who does all the work. She comes to class and immediately gets started on the warm-up. She turns in her all her homework and completes all of the assignments. Yet, despite all of her hard work, she doesn't do very well on the assessments. I believe she could be doing more to improve her grade, but she currently has a solid B in my class. What I am saying is that she should be getting an A.
I am wondering: Is that the way it usually works? I am trying to think back to my math classes in high school and in college. Actually, I probably shouldn't compare my high school classes. I was an awesome math student. In college, I tried al the homework and did about average on the exams. I think my grades in my math courses were influenced more by the exams than anything else. So, if that was the case in my math class, she would not be passing right now.
But that was college. This is high school. I do not plan on changing the structure of my class. I think her grade is a fair representation of her abilities and effort. I am showing concern because I am also her advisor. I do think that she is going to have to raise her grade though. Oh and she has a C- in guitar (her lowest grade). What's up with that?