Are you studying by practicing the material? For classes like Chemistry, I have to get practice sheets and do the problems over and over. Maybe you could even ask the teacher if she has any more worksheets she could give you, so that you can practice at home
If the test is multiple choice, I quickly do the ones I know and guess on the ones I don't immediately know (I put a mark by the ones I am unsure of and don't spend much time on them). When I have time, I go back and check the marked questions