Students will disagree with the way you graded an assignment or homework. First, determine whether or not you made a mistake. If you did make a mistake, correct their grade. If the student misinterpreted the question, explain the correct interpretation. If many students misinterpreted the same question, perhaps it should be thrown out. If the student made a mistake, explain what the student did wrong as carefully and politely as you can. If the student continues to disagree, direct him/her to the professor.