Skirts…
Basically tabris17 and PJR sumarizes it all. It is not necessarily the underwear we want to see, though, but rather the fact that something is covering it (the skirt) and the skirt has a, ahem, vulnerability (for the lack of a better word). Think about it: it is not the same when girls are wearing PE uniform, it shows the legs, but their undergarments are all, errr…, well hidden, without any chance that you can sneak on it. Nor it is the same a girl who is wearing a long skirt (ankle length) than a girl wearing a short skirt (way above knee length). We know that the former’s chance to peek is almost zero, while the latter, chances are greatly increased.
Likewise, it applies not only to skirts, but to shirts, too. If it rains, and the shirt gets wet, it will attract a lots of attention. Cases: Season of the Sakura, during the trip it rains and you and a girl look for shelter in a cave; Kakyuusei TV, episode 4, where Tsuyoshi peeks on Ryoko’s uniform after it got when during the rain. It also attracts lots of attention if the shirt a girl is wearing leaves some buttons, unbuttoned…
Wacky personal experience: During high school, there was a classroom that was oddly arranged. The instructor’s desk was a the opposite side of the blackboard, and there were three columns of chairs to the left of it and three to the right. The odd part was that, instead of facing the blackboard, the chairs were facing each other (that is, the the ones of the right were facing to the ones of the left and viceversa. Think about the neck pain we had after each class there…) The chair assignments were fixed by that instructor, and I was in the second column of the ones of the right. Nothing strange, right? Wrong! In the first column of the other side there was a girl who was a bit exhibitionist. Every now and then, she placed her legs in such a way that her panties could be seen! I didn’t ask for it, yet I had a hard time concentrating for the class during the whole bimester (when the instructor reassigned the seats). Also I was in the odd situation: should I have told her? I mean, if I told her she would have tought I was a pervert; if I didn’t, well, let’s say I managed to pass that class…