If the class is divided into X on average, the average score of one group is x+4, that of two groups is x-3, that of three groups is x-2, and that of four groups is x+ 1. Then the total average score of the four groups is [15 * (x+4)+13 * (x-3)+14 * (x-2)+12 * (x+1). So the total average score of the four groups is higher than that of the class.

Because the average score of the first four groups is higher than the class average score, and there are five groups, if the last five groups are equal to the class average score, only the fifth group is lower than the class average score.