Yup happened to me, so I stopped going to class and started self-studying. Ended up making much more significant increases on my own. My teacher was also poo, though, so...
Basically, it takes more than three tests to get used to taking a timed test, and it took me more than the duration of class to fully grasp the concepts well-enough to see improvements. Maybe you intake knowledge at a similar rate?
Don't worry, if your teacher is good, talk to him/her about your test performance outside of class. Don't accept the "oh you'll get better" appeasement as a response, push them for concrete advice on how you can improve.
Also, consider the PS Bibles -- they're pretty similar to TM strategies, and they helped me with fully understanding concepts that my teacher didn't explain too well in class.