When that happens to me I just forget all about it and just do the test. Assuming you go to class and listen, and take notes if there's any to be taken, when you look at the test it all comes back to you. For all of highschool I spent a total of 0 hours studying.
I do listen in class, but sometimes it goes through one ear and goes out the other :l
And since the class is in French, I only understand some part of what the teacher is saying..