High school is a time of change for pretty much everyone. I don't think anyone goes into 9th grade and comes out their senior year acting, looking, or feeling exactly the same.
I started my freshman year of high school as the model student. I had good grades, enjoyed playing sports and was quite popular and often accused of being "hyper".
I graduated from high school with a rebellious attitude, a weird haircut and an extensive collection of Goth CDs and band t-shirts.
During high school you're influenced by your peers, your hormones are going crazy, all you can think about is the opposite sex, there are exams to worry about, you say goodbye to old friends, you meet new friends, you have your first alcoholic beverage (despite still being underage), you're moody and confused and angry at the world - but also excited and adventurous and full of energy too.
With so much going on, it's impossible not to change. It's also a really important time of your life because it is the four years that "form" who you become and where you go next.
That's not to say you should be stressing out about it. If there's one regret I'll always carry around with me - it's that I didn't enjoy high school more.
Yes, exams and relationships and friends will seem like the most important things on the planet when you're in high school, but what I personally never realised back then is that things work out one way or another, and it's all about enjoying the ride and taking as many positives back with you.
RIght, I'm off to look at old yearbooks now!