Two groups of 12 students each took a math quiz. The mean score for group X was 85, and the mean score for group Y was 90.

Which of the following MUST be true?
I. At least one student in group Y scored higher than at least one student in group X.
II. Every student in group X scored lower than every student in group Y.
III. The median score for group Y is greater than the median score for group X.