From Blissful Sin:

After stomach cramps land Muramachi Koharu in a bed at the nurse’s office on the very first day at his new school, he suddenly wakes up to find himself on the landing just before the roof door. And there’s a student he doesn’t know sitting right next to him!
“Where am I? Who are you?”
The student says he’s one year Koharu’s senior and leans closer and closer to him…!
He pushes Koharu down… And suddenly kisses him!

A romantic comedy that tells the story of the faithful yet stupid senpai Oosugi and Koharu, a transfer student who’s too nice to say no. ♥

A 4-koma set in the "school" at the Alnus base.

"I've liked you for a long time. Please go out with me!" These were the words second-year high-school Kazuki Yugawara heard when one day out of the blue, someone confesses their feelings for him! But, wait just a minute… I go to an all-boys school. And the one who confessed to me was Sakurada-kun, who… is also a boy! I’m happy someone told me they liked me, but there's no way I could ever go out with a guy! But this guy Sakurada looks like a girl. What to do?

From Chibi Manga:
The kind homeroom teacher, yoshino sensei, is actually has a black heart! sensei is quietly meddling in here and there of suzuki's school life who accidentally find about his secret. tsundere high school girl x wicked teacher, secret lovey dovey school life!

A freshman in college, Amano, is taken a liking by a mysterious senpai, Kamishiro, who raises chicks. When Kamishiro hugs him out of nowhere in their first meeting, he thinks this senpai is crazy, but as he spends more time with him he sees how kind he is. But one day, while Amano is playing with the chicks, Kamishiro kisses him out of nowhere! An airheaded research student and a kuudere freshman's love story.

Hokuto Kuzuhara is a delinquent known throughout Sekai Private Academy. His way of life being so hated by his family that they give him an ultimatum: find a girlfriend within a year or move out. That said, though, months pass and he still has had no luck and it seems like he never will. It is then during this predicament that he is approached by the student council vice president with a plan that would both save him from moving out... As well as help her with her revenge.

Hito Natsu no Kids Game (ひと夏のKIDSゲーム Hito Natsu no KIDS Gēmu) is a short, debut manga work by Ken Akamatsu, the creator of Love Hina. The manga was published on September issue of Kodansha’s Magazine Fresh on September 10, 1993. Ken Akamatsu later received Kodansha’s "Freshman Manga Award" (Japanese: 新人漫画賞/少年マガジン新人賞) and Special Jury Award (Japanese: 審査員特別賞) for this work.The manga was re-published on 20 May 2001.

(Description taken from Wikipedia)

Someone is trying to kill Maejima, the unassuming Archery club adviser and Math teacher at Seika Women's High School. What appears to be pure, dumb luck has gotten him through several attempts unscathed, but he has no clue as to the perpetrator and the Principal would rather not involve the police in what's "surely just a student acting out". With the entire faculty and student body as his suspects, Maejima struggles to discover the culprit before his luck runs out.

Mamoru is a Nishiarai game player who has switched to a first-class powerful new high school named Hijiri Gakuen Koukou. There he met with a group of eccentric game lovers who call themselves Dennou Kenkyubu, he accidentally joined and had to undergo a variety of events and adventures with them.

The story starts with the death of Hikaru Mikado. That guy was a natural charmer, a “harem king”, but had no real male friends. After he died, he became a ghost, a soul, but Koremitsu Akagi can see him. Koremitsu though, isn’t really like Hikaru. His red hair and his evil-looking eyes make him look as a delinquent, even though he is very kind inside. Hikaru asks him a favour: he wants to fulfill a promise, which he couldn't fulfill anymore. The promise was to give his girlfriend and fiancée, Aoi Saotome, seven gifts for her seventeenth birthday. Koremitsu accepts, and has to work hard to help fulfill Hikaru’s promise with Aoi.

Spin off of Hitsugi no Chaika. Takes place in high school.

From Winter Translations:

This is a story about a pair of twins who're, undeniably very different from one another. Big brother Shuuya and little brother Yuuta are basically light and shadow...

Rika is known to be a cheerful girl, but is that her real self?

Hime Arikawa is a high school boy with a huge debt, and he is saved by the girls of his school's student council after being chased down by yakuza debt collectors. The student council agrees to pay off his debt if he'll join the student council and spend his high school life cross-dressing.

By all appearances, Sakura and her friends are normal, albeit sheltered junior high school girls who enjoy talking about love and walking home from school together. However, they also happen to inhabit a strange and supernatural world populated by mysterious, gigantic beings and unusual plant and animal life. Amid their peaceful lives, Sakura becomes very interested in something only alluded to exist in an old book: dicks.

A collection of stories:

• Otonari no Senpai
• Fuwafuwa no Kimochi
• Hajimari no Kotoba (also in Kirakira no Natsu)
• Hon no Tomodachi
• Mainichi no Tomodachi
• Christmas no Yoru (The Night Before Christmas)

In the casual days of the spring of his third year in middle school, Mitsuru Hibino meets transfer student Kouzou Issha, who develops his special power. Issha commands Hibino to use his lethal “power” to “correct the errors of this world”.
Their first step is to “kill those who drive while using their cell phones”. While getting their hands dirty with what society considers “crime”, these boys use their “power” for their “ideal justice”.

From Hearty Thrills:

Meguna Hirasawa has confessed 5000 times in total to her 10 year long crush Ayumu and in turn, received 5000 rejections. But that won't stop her. In fact, Meguna is retiring from simple confessions and moving to a whole new level. That being said, Meguna is ready to dive straight into seduction! Wait, the first thing she does is smash her lips into Ayumu's face for a bloody kiss?! Will her pestering finally allow her to capture Ayumu's heart? Or will this desperate girl's hard work be in vain?

From Chibi Manga:

You can't compare me to the perfect sister! I don't know any sister but when I entered in a far away high school something happened...

Collection of one-shots:

Chapter 1 - The Unripe Fruit
Chapter 2 - Notice My Love
Yoshihara has again bumped into Hoshina, his high school kouhai. Once, when they were both drunk, Yoshihara slept with Hoshina. Since that time, Yoshihara realized that he loved Hoshina, but unable to tell Hoshina his true feelings and unable to withstand the pain of a superficial relationship, Yoshihara distanced himself from Hoshina……. Aside from the titular story, this masterful anthology includes such short stories as “The Unripe Fruit” and “That Is the Name of the Flower You Cherish,” stories that are brimming with Narazaki Souta’s storytelling charm!!
Chapter 3 -That Is the Name of the Flower You Cherish
Chapter 4 - Pierce Me with Your Gaze!
Chapter 5 - HEAT ART BOYS