Well, first, you need to be confident and friendly. Try to start conversations with her about things she's passionate about. Also, show that you're a good listener and care about her feelings.
Be yourself and show genuine interest in her. Join activities she likes and try to have fun together.
To get a girl in high school, be kind and respectful. Pay attention to her needs and offer help when possible. And don't forget to work on your own hobbies and interests to become more attractive.