Well, to be a wallflower in high school, you might keep to yourself a lot. Stay in the background during class discussions and avoid joining any clubs or teams. Also, don't try to stand out with your appearance or behavior.
You could try to be quiet and avoid drawing attention to yourself. Don't participate in too many activities or social gatherings.
To become a wallflower in a high school story, start by being really reserved. Don't talk much to your classmates, and don't volunteer for anything. Dress in a plain way and don't show too much enthusiasm about anything. That might help you blend into the background.