As a six year old, Rintaro is diagnosed with Henisei Tashu Ijou Syndrome (also known as Hentai Syndrome), a disease that could kill him if he gets sexually aroused. To save him and provide a way for him to live as normal a life as possible, his father teaches him the ways to control his body and mind for ten years in the mountains, with the martial art of the Koei Sougetsu. When he's 16 he chooses to enroll Nangokuren High School, which he assumed to be the biggest all-boys school in the country but is actually the biggest all-girls school that has just gone coed.

On his first day he stumbles upon a large scale battle and learns that the first rule of the school is that it "Abides the Law of Nature" which has turned the campus into a war zone-like environment. Rintaro now sees transferring out as his mission, but will it be that easy now that he's caught the eye of the 3 "Dragons", the bodacious female leaders of the three biggest factions around?

A demon sets out to create an all-girls harem of girls to impregnate and save her species from extinction.

A powerful grand wizard named, “Raim” reappears after a millennium because of a failed experiment. While sets out to regain his lost tower, he meets “are” and “ere” who have been trafficked as slaves... a combination of a weird grand magician who only uses his wits to make money and the cute, lovable brother and sister of the miao tribe! Their endless adventure is beginning to unfold. Editorial staff’s comment Will Raim be able to regain his lost tower?! 

Manhwa you may like:
+ This is another ambitious work of Sung San Young, The Gamer
+ Tower of God
+ The God of High School

As a direct continuation of Mujaki no Rakuen, we see Shouta in even more troublesome and various situations.

As a guy with no real friends, Nishina Tadatora has spent months just observing Ichinomiya Misumi who sits next to him in class. At first glance, Ichinomiya looks like a lonely girl without any special appeal, but only Nishina knows how sensually provocative she can really be. Upon realizing that Nishina is watching her, Ichinomiya begins to interact with him in her own unique way. But unbeknownst to Nishina, Ichinomiya also seems to be harboring a secret...

Hida Kizuna possesses the HHG (Heart Hybrid Gear) ability, but it is not strong enough to make him particularly important. His older sister calls him to transfer to a strategic defense school, where many of the students (many of which are large-breasted girls) use their HHG abilities to fight invaders from another world while wearing extremely skimpy pilot outfits. Kizuna's fighting ability doesn't measure up, but his sister has another plan-- apparently having erotic experiences with Kizuna will allow the girls to replenish their energy or power-up. It looks like his new school life is going to be full of embarrassment.

Senou Natsuru is a normal high school boy. One day he wakes up to find himself turned into a girl. A stuffed tiger "Harakiri Tora" comes to life and tells him to fight as a female psychic fighter "Kämpfer". Having no idea about what's going on, Natsuru is involved in the battles against other "Kämpfer".

Kämpfer is German for fighter.

From Yen Press:

Shouta Yamakawa is in love with manga…and the 2-D girls in it! So when a casually uttered wish to have the lovely, frail Sora Nagihara-his favorite comic heroine-by his side comes true, Shouta can’t believe his eyes! But he quickly learns that while this Sora is just as cute as the one in his comic books, “frail” is the last word that comes to mind for her personality! She’s one bold, brutal babe, and she’s ready to bring the world to its knees for him, her new master?!

Failed on his job-hunting, shut-in Sagara Kurohiko went to the mountain on the day of a typhoon where he is struck by lightning and dies. But he is still alive, in a strange land with a great holy tree. To his surprise, his physique has rejuvenated to when he was teen. Brought to the academy near the tree, he is soon suspected as a spy from another country, and was brought to the headmaster. An accident happened on the way, which enraged the headmaster. Before Kurohiko can get blasted by magic, his eyes laid upon an old parchment. He took it and read the contents. Suddenly four holes appeared and black chains came out from each of it, binding the headmaster. It is then revealed that Kurohiko just recited an incantation of "Forbidden Art," an art which is extremely powerful from a legend. Apparently, no one has been able to decipher the contents of the scroll, which makes Kurohiko the only current Forbidden Spellcaster. Sparked by this, the headmaster offers Kurohiko to attend the academy.

Lee Wee Jin has to cope with his tyrant sister on a master-slave basis everyday. Trying to fool himself he evades reality, thinking she's tsundere and that she really likes him. Suddenly, these delusions become all too real.

Seto Yu is a 12-year-old boy, that is abused violently by his father. One day, he is transported into a fantasy world with video game mechanics after being killed by his abusive father and he discovers that he has the unique ability to steal other people's skills for his own use. He quickly uses his special power to take revenge on the townspeople who bully him and obtains a harem of beautiful girls who love him unconditionally even though he shows no romantic interest in any of them and has the physical build of a skinny teenager

Hyoudou Issei is an ordinary yet lecherous high-school student who is killed by his girlfriend, Amano Yuuma, during their first date. Yuma is revealed to be a fallen angel named Reinare who was sent on a mission to eliminate divine weapons. Issei is later reincarnated as a demon by his senpai Rias Gremory; in return, he will serve under Rias to fight against the fallen angels.

The protagonist is a guy who is among the stupidest of the stupidest in the school. In this school your grades can, under the supervision of teachers, take a virtual form to do battle! However, if you leave an examination midway, you get zero marks. A bright and cute girl, Mizuki Himeji, had a high fever during her examination. Despite her potential to be the second highest scorer in the level, she obtained zero marks for having left due to her illness and is thus banished to the worst class, Class F.
Grades mean nearly everything. For Class A: They have a smart-looking teacher, a plasma TV as big as the entire wall as their blackboard, personal laptops, personal air-conditioners, refrigerators, adjustable seats and all kinds of different appliances. Within the refrigerator, there are all kinds of drinks and snacks. Their ceiling is made of glass, the wall lets them put up high-class drawings and plants.
As for Class F (the worst class): They have Japanese desks and seat paddings. Their blackboard is dirty and there's even no chalk! Someone complains the legs of his desk are broken, so the teacher asks, "Didn't we distribute some glue to stick wood together? Glue it back yourself later." Another one complains the wind is blowing in from the broken window. The teacher says, "I got it. I'll apply for plastic bags and super-glue to fix it later." There are spider webs everywhere, not a patch of the wall is clean. A unique moldy smell spreads across the entire room; it must be from the old tatamis used to tile the floor. Welcome to Class F.
The protagonist feels displeased at such an arrangement; he seeks help from his friends to try to make a change through invoking war! And they devise all kinds of plans to try to beat the famous Class A, scheming to get Class A's classroom and equipment if they win.

Hachimitsu Private Academy was a revered and elite all-girls’ boarding school on the outskirts of Tokyo…once upon a time. But with the new school year comes a revision to school policy: Boys are to be admitted into the student body for the first time ever. But on his first day at Hachimitsu, Kiyoshi Fujino discovers that he’s one of only five boys enrolled at the school. Their numbers overwhelmed by the thousand girls in the student body, is it heaven or hell that awaits these five (un)fortunates?!

Note: Was one of two winners of the Best General Manga award alongside Gurazeni at the 37th Kodansha Manga Award in 2013.

Tezuka Kanna, 15, is a girl who loves manga. For that reason, she specifically enrolls in a course for manga in Tokiwa High School as a freshman! No one knows though, that she is already a debuting manga artist. ...But wait, what's this?! Three handsome guys suddenly appear, and what's more they know!!! Kanna (God) vs. Those Guys (Devils)!!! What would happen next?!

Taichi Yamada is what you would call a fan but he is not just any fan, he is a fan of Kokoro Shiina, a gravure model. Maybe it was fate that brought them together that day but Taichi met Kokoro when she literally fell from the sky. Waking up in the hospital, he begins to see Kokoro's spirit and after roaming the hospital premises a "shinigami" arrives to take Kokoro to the underworld. Is this the end for Kokoro? What will Taichi do to save his beloved idol?!

Seven years ago, Yuto's older sister vanished through a mysterious door, leaving him with only a necklace. Now he has once again seen that same door, and watched two mysterious girls (Saya and Hina) go through it. Eager for clues about his sister's disappearance, he and his pet cat follow them through. However, he soon finds there's a reason his sister wouldn't let him come with her. In the deadly world on the other side of the door, his ignorance leads to tragedy. Now Saya insists that he travel through the door once again, to search for a way to retrieve the lives that he caused to be lost during his first journey.

From Seven Seas:

Welcome to Ansullivan Dragonar Academy, a school with a difference: the students are dragon riders, each contracted with their own personal dragon. Ash Blake has the unique ability to ride anyone's dragon, but he's been waiting for what feels like forever for his own dragon to be born. He's thrilled when his dragon, Eco, finally appears--but she inexplicably takes the form of a beautiful human girl.

Eco's peculiar appearance is only the first unusual thing Ash faces. His life is about to become a whirlwind of spies, dragon races, and unexpected attention from both the proud Princess Sylvia and the beautiful president of the student council, Rebecca. All Ash wants is to acclimate Eco to the world, but someone out there wants his strange dragon on the dissecting table!