In the conclusion of a persuasive essay, it is standard, good practice to restate your thesis (your statement of position), summarize the points of persuasion that you discussed within the body of your essay, and then, in an essay like the one you are writing, to end with a call to action. The purpose of persuasion is to cause a change of thought within the reader or have the reader actually DO something. If your paper is dealing with persuading teachers to assign less homework, then I'm assuming that your audience (who is is that you intend to read your paper) is teachers in general. A good conclusion, then, would ask them to do something - in this case, give fewer homework assignments. The answer from ashwang incorporated this, as well as another highly effective technique for persuasion, direct address, where you speak directly to the reader. A good concluding sentence, then, would be a call to action using direct address.