The test scores of twelve students from two different classes are shown below. The mean of class M's scores is 77 and the mean of class N's scores is 80.

Which of the following statements MUST be true?
I. There is at least one student in class N who scored higher than at least one student in class M.
II. The highest score of class N is greater than the highest score of class M.
III. The mean score of class N is higher than the mean score of class M because every student in class N scored higher than every student in class M.