Prewrite to sort out your reasons. Writing this essay off the cuff will likely leave you rambling instead of making a clear argument as to why your grades were less than perfect. Create a web to help sort out your ideas, writing "low grades" in the center and placing potential reasons why surrounding it. After creating your web, evaluate your reasons and decide which of them seem the strongest and therefore should be covered in your essay.
Introduce the essay by stating the importance of grades and acknowledge responsibility for your grade issues. In your introduction, make it clear to your audience that you see grades as important and that this essay is not an attempt to skirt responsibility for your previous grade issues.
Order your reasons from most to least compelling. While all of your reasons should be strong, some are certainly stronger than others. To ensure that your essay catches your readers from the beginning, place your strongest reasons front and center.
Compose a paragraph about each reason. To make your arguments strong and convincing, elaborate on each item. For example, if one of your reasons was that you experienced family turmoil, explain what happened. By doing so you will increase the likelihood that your readers empathize with you and agree with your arguments.
Conclude with an explanation of your grade goals. Explain in your last paragraph what you want to accomplish now in terms of grades. By ending your essay with your goal, you allow readers to see that these negative grades are a thing of the past, not something you hope to continue into the future.