A really messed up, sadistic one! It's called Kangoku Gakuen; "At edge of Tokyo, there was a Hachimitsu private school, it was a honored all girls boarding school...at least it used to be. From this year, because of board chairman has been switched, that education policy has been changed. It allowed boys to attend the school but only 5 boys..."
The one before that was Kono S o, Mi Yo!; "22-year-old Rin has two major problems: a complex about the tattoo-like birthmark on his butt and a crush on an older woman who thinks of him as her little brother. Rin hasn't allowed any woman to see his naked bottom since an incident when he was 7, which firmly entrenched his shame about his birthmark. When he finds his birthmark may grant him mystical powers over the opposite sex, will he finally be able to use his butt to his advantage?"
One before that was Ibitsu; "There is something a bit twisted inside of the teenage girl Madoka. It comes out one day when she has to deal with a groper on a public train. He may be scarred for life by the time she's finally through with him. But for her, at least, this encounter may only be the beginning..."
Kangoku Gakuen is really messed up and sadistic... but for some reason I can't stop reading it. The guys decide to peek on the girls during bathtime and get caught, they get thrown into the schools prison and are repeatedly beaten and tortured and humiliated... you wouldn't like it~
Kono S, was quite ecchi filled but really well done story, nice artwork and a good romance~ Lots of twists and messed up things... but it's ongoing and I got caught up. You might like this one... but the amount of ecchi would be off-putting for you.
Ibitsu, it's quite messed up also and from the same author that wrote Sundome... which was pretty messed up... it's quite ecchi too... deals with bondism and various sexual fetishes... though it doesn't really show anything, it's still ecchi in nature. I stumbled on that one by accident and read it all, it's still ongoing~
I started reading Yakitate Japan but couldn't get into by chapter 15, so I decided to download the anime, I'll watch that~ My cousin said the anime was really good~
/edit - I got bored and wrote a little something, tell me what you think?;
http://fav.me/d6gkucu