I can hear you saying now, "Peter what is this and why does the title make it sound like it has to do with masturbation?". The answer to these questions is "It is a fantastically good manga series from 2000, and the title sounds like it has to do with masturbation because it does. Specifically, it's about a guy who stays after school every day so he can sneak into the girl's washroom and jerk off."
BUT IT'S ACTUALLY REALLY GOOD, I SWEAR
Every day, after classes are over, Kurosawa Kakeru kills about an hour in the library to give the school buildings time to empty out, then sneaks into the 3rd floor girl's washroom to masturbate while fantasizing about one or more girls in his class. Once he's finished, he cleans up and carefully sneaks out. He's been doing it for a while, and he's got a pretty good system down to get him in and out undetected. Until one day, near the end of the school year, he bumps into Kitahara Aya, a mousy, shy, bullied girl, on the way out. He makes an excuse and brushes it off, sure that she didn't suspect what he was doing and even if she did, she wouldn't tell anyone.
Next year, last year of Middle School (I think), he winds up in Kitahara's class. After seeing her get repeatedly bullied and picked on, and following a particularly repugnant incident where she was framed for stealing the fattest kid in class' uniform, and forced to falsely and very publicly confess her love to him, Kurosawa decides that he'll get back at her tormentors for her, in what he considers truly poetic justice: He steals their gym uniforms from their lockers, takes them with him when he goes to perform his Daily Routine, and...well, you can imagine. He feels very smug, since he got away with it, and is convinced that he was serving out some sort of twisted justice.
Then there's a knock on the stall door while he performs his daily ritual. It's Kitahara. Not only has she figured out that he's been doing this every day, but she knows that he was the one who fired back at her tormentors on her behalf. And she won't tell anyone...so long as he does what she asks. And what she asks is for him to help her get revenge on all the other people who've wronged her.
I KNOW, I KNOW. It sounds like the set-up for a bad piece of pornography. BUT IT'S REALLY NOT! In fact, it's not porn at all. The closest it ever gets is Kurosawa's fantasies, which still don't actually show anything through the magic of Convenient Camera Angles, and aren't nearly as common as you'd think. If you read this manga for titillation, you'll be sorely disappointed.
What is is is a story about growing up. It's about the highs and lows a person can and will reach in their lifetimes. It's about the importance of friendship. It's about learning to see women as people rather than sex objects. It's about taking responsibility for your actions.
Although Kurosawa starts off the series as a smug prick, practically lauding himself as a hero, as the series progresses he becomes increasingly uncomfortable, not only with his own actions, but also with Kitahara, who, like any hurting kid given a weapon, is lashing out at people all around her, even those who haven't done her any real harm. He changes dramatically through the course of the series, as does she, through contact with each other and witht he impromptu group of friends they find themselves a part of. It's really very powerful, and I really recommend at least giving it a shot.
WARNINGS: Uh, well, sexual content, obviously, although like I said above it's not terribly explicit. Some of his fantasies (OK, all but...one...of them) are pretty rape-y, in the grand tradition of hentai. Oh and there's some stuff late in the series that could be triggering in terms of self-harm stuff.