This is an interesting take on a genre that has always fascinated me and, to be honest, is a bit of a fantasy of mine. (Yes, I am a closet exhibitionist) The set-up is intriguing, a play off of the boy looking up the schoolgirls' dresses theme.
It is very well written, although the PoV tends to switch back and forth from person to person with little warning in the erotic parts near the end. The only nit that I have about the story is that there are some parts that feel like they are missing. Can't exactly say what they were, only that they involve the 18 year old girl. I would have liked to seen the discussion with her go into a bit more detail, as well as her reaction when she sees him naked.