From whyanime:

This series is about a girl named Otome who's searching for love now that she has entered high school. But of course, finding love isn't going to be easy, especially if you have three delinquent boys who are your childhood friends. Otome explains to the boys that since they are in high school now, she wants to find love and fall deeply in love, but Tokio (one of her childhood friends) insist that she be part of his plan. He dreams of taking over the school and becoming a grand gang leader; he'll even let her join as the sub-leader. What is she going to do? As she sneaks away from the boys, she catches a couple of girls watching the most popular boy, Touyou, being confessed to!? Is this love at first sight? She decides to spy/stalk him... what will she find out?

Extra 1 - The Boy Next Door (Tonari no Danshi)
Extra 2 - System vs Freedom

From Yen Press:

Welcome to the prestigious Saint Leaf School, where, among other things, the boys are so pretty they give the girls a run for their money! Amidst this bed of flowers exists one Hitomi Sakuragawa, a sweet girl with a not-so-little weight problem, who lives in a nearby apartment owned by her doting older brother. Hitomi also happens to have pretty much the best luck on the planet: Her neighbors are some of the hottest guys in school! But will she need to steel her willpower, cut out the cookies, and slim down before they’ll even give her the time of day? Or will they be charmed by her kindhearted, genuine personality? It won’t be long before Hitomi discovers that the road to a healthy lifestyle is paved with hilarity!

Tachibana Otome, a 16 years old otaku whose type of guy was "Blonde Hair and Blue Eyes" suddenly meet the that sort of person in real life....... How will it go for her?

1. Baumkuchen*Record
2. Antenna Kaidan
3. Vinyl House
4. Umineko no Soup
5. Otomeza Spica Shinjuboshi

Akira is a young girl who, while not a recluse, is incredibly awkward when it comes to socializing. She desperately wants to make a friend, but it seems like it will take ages before that will ever happen.

This is a side story of the Tough series based around Miyazawa Seiko (Kiichi's father) and his brother Kiryu. Many of the same elements of Tough can be found here, such as mixed martial arts and a lot of violence.

Even though Mitsuru has a pretty face, he is followed by rumors about his promiscuity. In reality, he goes out night after night looking for strangers to have sex with. Rumors are flying around school that he is a flirt, but this is convenient for Mitsuru who doesn’t want to make friends with anyone. However, new transfer student Hasegawa is irritated by this attitude and he begins to follow Mitsuru around. Will Mitsuru’s loneliness be revealed as Hasegawa crashes into his life and stirs up his heart?

From Bliss:
Serious and kind, Noboru-sensei is a favorite at Atami Preschool. Arima-sensei is the gentle and lazy “puppy” who adores Noboru-sensei. Everyday Arima-sensei expresses his love whole-heartedly, but Noboru-sensei refuses to recognize his affection. Will Arima-sensei have no choice but to become a wolf to make his love accepted…!?

From Vices and Devices:

Yumeji has a big problem, and no, it's not that he's gay (for now, anyway). Despite his good looks, excellent grades, and talent for sports, he's not getting any, and it has everything to do with the fact that he's impotent thanks to a traumatizing event back in middle school. Nothing seems to help, and since Viagra is apparently not a viable solution in Japan, he's practically lost all hope--until he meets underclassmen Seiji, who's violent, tactless, and says whatever is on his mind. Conveniently enough, he's also in love with Yumeji.

3. Koibito Kijunchi
Mu's in love with his best friend Maru, but Maru doesn't take him seriously because, well, Mu's a huge slut who will sleep with anything that moves. So Maru suggests an "endurance contest": he'll sleep with Mu if Mu can hold out for ten days without sex.

From Tokyopop: Natsumi Kawashima, an energetic fifth-grader, is obsessed with her fantasy of being the ideal, sexy woman. Rejected at every advance by the cute guys that she fawns over, she finally gets a break when she picks up a package of mysterious pink nuts that are adults only. Eating these nuts causes her to morph into a voluptuous adult, and she's offered modeling opportunities and suddenly becomes noticed by guys who wouldn't have given her the time of day before. But, she's still a kid inside and is about to learn that things don't always work out exactly as you hope they will...

From Lovers Dream:

Alone and dumped outside of a love hotel, Mitsuka finds herself caught by the last person she wants to see--her school counselor and teacher, Kaga-sensei! Instead of turning her in, Kaga-sensei wants an interview. It turns out he's an erotic novelist, and he's looking for just the right dose of high school girl reality for his latest work. Kaga makes it clear their relationship is strictly business, but can Mitsuka keep it that way when she's falling for her teacher?

Volume 1 includes extra story: Then, Will You Fall In Love With Me?

Volume 2 includes extra story: Money Makes Being In Love Go Round

From Starry Sky Scans:

“What an erotic face, don’t be so greedy.” Shinoki has ended up working in the same company with his long-time rival from high school, Nagase. As usual, Nagase manages to catch everyone’s attention, irritating the hell out of Shinoki. At a drinking get-together with their colleagues, Shinoki ends up wasted and is sent off to a hotel where he spends one night with Nagase and gets fucked?!

Six months ago a book store manager taught high-school student Minami a lesson about loitering and gave him a job. Since then Minami finds himself slowly falling in love with his strict and sometimes violent manager.

From Fifay.net:
Naoto is a stressed-out college student doing his best to juggle the eccentricities of his divorced parents with his own burgeoning romance. His stress deepens after his gay father proposes to the handsome and elegant male lover. This is a humourous series by Ima Ichiko about the modern, dysfunctional family.

From Shoujo Crusade:

1. Otona no Shoumei
Tsubaki always appears experienced, but actually, she has never had a boyfriend even though she's already 16, and she's a real baby. She wants to quickly increase her love experiences, and become an adult! One day, she got called out by her math teacher, Shiina-sensei, and she was told that she'll have to take supplementary lessons. All alone in the math room with her sensei after school, Tsubaki's heart beat like crazy when she saw Shiina-sensei's facial expressions that were different from usual...?

2. The Imprisoned Princess
My hard work's finally been paid off! All that studying have invited me to "ouji-sama's" house! I couldn't be more happier! But wait, why do I have to clean his house?!

3. The Prince's Mantle
Continuation of the second story. Ouji-sama and I are now a couple! Many may admire me, but he is indeed not a 'real' ouji. I still love him though...

4. The Upside Down Message
I have always loved my childhood friend. Every valentines day, I send him "chocolate messages". But, he has a harem of girls? Without thinking, I told him I like senpai, but the one I really love is him...

After Shiori loses faith in her piano playing, she meets a weird Physics Professor who asks her to teach him to play the piano.

As Asuka dreams of marrying a wealthy man, she has polished both the appearance and mind. Now it is rumored that she is the coolest beauty in the secretary division of the company. However, one of her friends ran away with the elite who she wished to marry.

She had a drink in desperation, and slept with Yuroki, a nephew of the president of the company. Meanwhile, Yuroki is assigned to the secretary division, and the delicate relationship between him and Asuka begins. Asuka who is naive, though she is a capable woman, and Yuroki who is capable, though he is a greenhorn, develop a sweet love in a mature way.

Also contains a oneshot called Love Your Way.

From Royal Alliance:

Sensei, please teach me about love.
Senou Miku is a normal high-school girl. On the day that she had her first boyfriend, her mother had a job transfer. And then, her guardian, who came to live with her, turned out to be her homeroom teacher, Nishizono-sensei...

Otona to Kodomo no Kyoukaisen (roughly means A Man-Child's Boundary Line)

From Peccatore Sanctuary:

Tohru is confused and in deep trouble. After the 'thing' with Takafumi - a senior high student, Tohru can't get him out of his head. Tohru swore that he will wait until Takumi is legally an adult he will not do 'H things' (perverted things) with him. However, both are finding it hard to keep their urges under control... this is when the line between boy and man becomes conveniently blurred...

A one-shot webcomic with the existence of Santa as its theme.