Longitudinal change in the average Z scores for four groups of pupils sorted by quartiles was examined for its stability over three years. The data, collected from 1998 to 2009, was obtained from nine cohorts of Japanese junior high school pupils totaling 1,962 subjects. It showed illusionary declines among the mid-range pupils but improvements among those in the low-range group. These illusionary declines and improvements were found to stem from the fact that left-skewed distributions for the first term examination scores became less skewed for later exams. Such illusionary declines, dubbed the "leading group effect", may undermine the motivation of middle-range pupils. This illusionary statistical phenomenon should be appropriately explained to teachers and pupils of junior high schools so they won't be discouraged by illusionary declines in the Z scores.