Please, do not make hasty decisions, or simply follow what your friends are doing. From experience, poorly planned decisions very often do not work out. Of course, you are not alone in this important decision-making process. Speak to your parents, siblings, counselors, teachers, friends, and anyone who is going to give you some concrete advice. Yes, do well in all you do.
Every year is important.
Grade point averages do not start Junior or Senior Year. Your GPA begins the first day you enter school. This Standardized Test, fortunately, or unfortunately, is a big part of the college acceptance process. Whether it is your first year or last, have some fun. Get involved in sports, clubs and other activities.