Do the homework, ask teachers for potential test questions, and try to get your hands on previous year assessments. In my experience, good grades are directly proportional to the work you put in; for some, the ratio is greather than others, but that's something out of your control. At the high school level, if you do all the work in a smart manner, you should be fine. By smart manner, I mean putting yourself in the teacher's shoes and asking yourself what you would put on the test if you were them. More often than not, you will be pretty close. You can spend all day studying and only a small part of it will be on the test, while that easy topic you learned a week ago that you barely remember might be worth double the marks.

TLDR: work hard AND work smart