High school is one experience that changes everybody. I have seen quiet people suddenly become talkative and nice girls become, well, not nice anymore. You may not realize it, but you changed too. Think of how you were before high school and than after high school. You wouldn't be able to recognise yourself anymore.
Sometimes you have to change or else you will be consumed by yourself. Ww mostly change to survive. High school is totally different from elementary school and jr. high school. The classes are harder and the teachers are meaner. The people in the hallways will ignore you. Your friends may not really be your friends. Picking friends can be a challange. If you pick the wrong people, your whole high school experience will be ruined. Choose people who like you, not what you wear. Choose people who listen to you, not insult you. If you get this right, high school won't be as bad as you first thought.
When you graduate from high school, you will have matured. The way you think will be different than a jr. high person thinks. The problem with that is will forget what it felt like being a kid. You will treat kids the way adults teated you. The cycle will continue.
All the people you leave behind in high school will make you realize how much your friends really mean to you. Don't let go of the people you can't live without. They are your gift from high school.