The real key is this: at the end of the class do you think you'll have learned the material you'll need in life? If you answer 'yes' then the instructor and students are doing just fine - regardless of test scores. If you answer 'no' then I usually put the blame on the student. Why not the instructor? Well as a student you need to learn to deal with multiple types of people - good and bad - including instructors. If you get a lemon for an instructor, you need to take the initiative and read up on your own, post here for questions you have, and basically learn the material outside of class. If you don't do it, then you are a bad student and it is your fault if you don't learn.
Back more to the topic. Low test scores are a sign of a great class to me. I've had classes where 10-20% was a D, 20%-30% was a C, 40% to 50% was a B, and 50%+ was an A (Note: a SCALE - which I love - has nothing to do with a CURVE - which I detest - but that is another thread). Damn those were hard tests. But guess what? We all studied extremely hard, got our 51% to scrape by with an A, and learned a whole lot from the class. Sure I hated those, but they made me a much better person after they were done.
In classes where everyone in the world gets an 95% or above, then you never study, and you never really learn. Then it is the instructor's fault. His/her policy lets everyone skip class, not do the homework, etc, yet still get an A. High scores thus signal bad instructors who's students don't learn a thing.