What i want to know about my subject

What I want to know is why do they make us take classes that just teach us the same thing over and over again. Why do they not let us try and take a test that would allow us to test out of the class that way we could further our education faster and be able to get to work faster and improve our society? When the teachers see that a student already knows something why cant they offer a test that will allow said student to be promoted to a higher grade or class. Thus cutting school time down and that would reduce cost of teaching the student and providing materials that the student would have to use while in the classroom even tho he or she doesn't need it?