Everyone in school thinks I am bitter. How can I change their minds?

First of all, don't live based off on people's opinions of you. Do you think you're bitter. If yes, then that's when you'll need to do something. First, I think you should try to keep a relaxed face, if you don't like smiling. Let your guard down a little, don't just build walls around yourself. Try to be more approachable. Let people feel like they can actually talk to you. Smile more. Don't force it. Just try to make yourself happy most of the time, think good thoughts, and you'll be smiling a lot. This is all my opinion though

If that's what they think then that is their fault, they don't know you well enough to actually think that about you and to be able to have a good opinion of you that's true, you don't have to change yourself for people, and especially if they are being toxic to you. To help this and change people's minds find good friends that will accept you for who you are and don't think that about you. If you think about all the negative things that are said to you then think about all the positive in your life. Remember you are worth way more than what people tell you you are. :)