for real read the textbooks, more helpful than the teacher the majority of the time because it doesn't miss any gubbins and is straight forward.
(at least for Canada, I don't know how different or good textbooks are in the states.)
In my highschool the textbooks were long ways to learn something so my teachers basically give you a short version of it and it did help a long way.
Every highschool is different about learning so maybe find some research on the school you are going to, might help you prepare for it.
Later on in your years take the ACT or SAT if you plan to go to college, I regret not doing this. Same for scholarships, apply for those too in your later years.
Obviously pay attention in class, there is a chance something might be on the test that only the teacher said. I rarely did homework but still did good on tests, but if a class relies on homework a lot, just do it for the higher grade.
Try to get into more sports and clubs if you have not yet, you can get popular that way and find people if you need help with stuff. I have been on Track for 3 years and went into a few tech clubs, made a bunch of friends that were pretty smart.
Also note that whatever you do for the most part in your classes will go on your transcript, so try not to do anything bad or mess up your grades, if you plan to go to college.