I still remember the nervousness I felt in 8th grade a year before I became a freshman in high school. I felt scared high school would be hard or there would be mean people. In reality, there was nothing to be afraid of and I came to find out that high school can be some of the best years of your life. People older than me always told me I would miss high school when I went off to college and I just kind of laughed at them. As I am a college freshman now I understand what they mean.
Get involved as much as you can, this is something I regret greatly. I wish I would have continued volleyball and cheerleading. Most of all I wish I would have joined jazz band and FCCLA my senior year at my new school. Don't ever let someone persuade to not do something. If you do have these negative people clinging to you getting you to not succeed let them go. I wish I would have realized this in high school that I needed to let go of some friends that didn't support me. In college, you will also learn you just can't fit everything you would like to in your schedule. I participated in band since 5th grade and I had to let go of band with having 18 credit hours my fall semester of my freshmen year of college.
Get a job because it'll make you responsible and get you money, but don't overwork yourself. I worked at Wendy's from my spring semester of my sophomore year to the summer before my senior year. I loved that job as a teenager trying to pay for a car, but I had to quit because of moving two hours south. I was a great worker, but looking back I worked myself way too much while trying to be a high school student as well. I worked 30-40 hours a week and attended high school. Anytime someone called in I would cover that person's shift yet whenever I needed someone to cover mine I couldn't find anyone. Something I wish I would have learned is that it's okay to say no. Although I don't regret being a hard reliable worker because it showed how great of a worker I was and got me some great raises.
Make as many memories as you can with your friends because you will all probably go your separate ways heading into college. Make time for being home and spending time with your family, though. I always looked forward to moving out and having that freedom, but I promise you it's not as great as you think it is. Even though you may not want to admit it you will miss home. Enjoy eating home cooked meals because most college food sucks. If you don't share a room already enjoy having your own room and not sharing your bathroom with 50 or more people.
Just live in the present and don't wish away your senior year.