Well, you certainly are not in a position to write -- not yet at least. First, sit down with a pencil and paper and write some ideas -- about your life, how you have lived your life thus far, how you handled high school, or any other situation in your life. You might just find that your ideas about high school are not strong enough and that you have another idea about an essay. Who knows? Only you do. But, you have to start by just writing some ideas. Don't worry about the spelling or the grammar, just write something.
Then, when you feel as if you have enough information on paper, start writing about a specific time in your life when a mistake cost a lot. It might just be high school; on the other hand, it might be something else. Brain storm. Do you understand what I am trying to tell you?
You can then begin by developing a thesis statement -- a statement that identifies your thesis. Build your essay around that, use paragraphs to separate ideas, and use a topic sentence for each of the paragraphs.
Post your initial essay on this forum. We will get back with you and let you know how well you are doing.