But how often do you see your school friends in normal clothes compared to school uniform?
Summer uniform usually has thinner fabric. Combo that with sweaty body and sometimes you might get a glimpse of whatever under those cloth. Moreover, the slight armpit window that you can sometimes see when the girls raise their arm around shoulder level.
In addition, we are talking about horny teenager in the peak of their hormon growth plus the fact that their female classmates are also undergoing or even finished their puberty, so you can say that it's their first time having a face to face with a woman body.