Fifteen years ago, natality decreased greatly in Japan and the few who were born during that time are called the "Old Generation" or "Oldgens."
Hanasaka Haru, a 15-year-old Oldgen, has never met any classmates before. Now, he goes to school for the first time and meets Himeshiro Sakurako. But Sakurako is not quite the ideal classmate he imagined, and his dream school life isn't turning out the way he wanted it to...

I love math. Just after the high-school entrance ceremony, I meet a beautiful girl: Milka. Milka is a mathematical genius. She shows me many math and many elegant solutions. Milka and I spend a long time discussing math in the school library.

One year later, I meet another mathematical girl: Tetra. Tetra is one year younger than me, and asks me to teach her math. While I teach her, she begins to understand math and gradually to love its elegance.

The third series is about logic puzzles, Peano arithmetic, epsilon-delta, Cantor's diagonal argument, Hilbert's program and Gödel's Incompleteness Theorems.

From MangaHelpers:
Yasaka Tokiwa lives in an unusual time, and according to his dreams, it's about to become even stranger. He's been having a dream of a woman called Ri'in ever since the robot, who he calls Makina, came to his home. He and Makina attend the same school, where their other classmates include a secret ninja (Kusanagi Kanata) and a sorcerer (Yata Haruka).
When Tokiwa, the robot, the ninja, and the sorcerer rush to rescue the student council president from a demon, the boys find that they've all been having the same dream! What is going to happen to their world?

Jang-Mi and Taebong's long-time friendship is put to the test after they both sleep together for one night. Is this love or just sex? Read H-mate ( Hメイト, H메이트) by Turtle Feet now!!

Maybe you'll like: 
+ Brawling Go Manga
+ Skill of Lure
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+ H Campus
+ Yahalue
+ A World That I Rule

From Kodansha:

Class troublemaker Ryu Yamada is already having a bad day when he stumbles down a staircase along with star student Urara Shiraishi. When he wakes up, he realizes they have switched bodies—and that Ryu has the power to trade places with anyone just by kissing them! After figuring out the workings behind this new and amazing ability, Ryu and Urara take full advantage of the situation to improve their lives. But with such an oddly amazing power, just how long will Ryu and Urara be able to keep their secret under wraps?

Once upon a time there was a loser, but that one day he got the ability. Ask what? Well ... it's hard to explain, he knows how to instruct on "the right path" girls who strayed from it.

From Easy Going Scans

What happens when you take a good, long look and realize that your boyfriend is prettier that you? And what happens when you go to a high school with so few girls that you're considered a goddess? But then again, your pretty boyfriend is also attending the same school...

Minato is a total loser. He's failed to make it into college twice, he's unpopular with women, and he's unemployed. He seems like the classic-useless-shounen-harem-male-lead...
In truth, however he's actually an Ashikabi--one of a mysterious set of "masters" in an awesome battle royale between hordes of cute girls called "Sekirei."
Since this is an ecchi manga, with their attacks they often happen to... you guessed it--ripping each others' clothes off! Looks like his luck's taken a turn for the better.

Nanami Rei has always lived his life in his brother's shadow. He can't talk to girls, he's not athletic, and his grades aren't good. On the contrary, his brother Ren is smart and popular, but he has never judged Rei for his faults or lifestyle. One day, Ren goes into shock and mentions to Rei the name, "Wonder Rabbit Girl". Rei, seeking to find this mysterious figure, attends Ren's school, disguised as Ren himself.
Rei comes into possession of his brother's USB drive, which contains a list of the school's girls and their fetishes. Rei believes that one of these girls must be the Wonder Rabbit Girl.

When humanity was threatened 5 years ago, the boy who saved the world was driven to despair and became regarded as the enemy of all of mankind. And now a new crisis approaches humanity. The curtain rises on a gallant drama about a hero falsely charged.

Ishikawa Takuya, a high-school student who doesn't stand out in any particular way, is surprised when his principal asks him to take care of some transfer students. He is even more surprised to find that there are actually 30 charming female transfer students in his care, who have come from a boarding school that was destroyed in a fire! At first it seems like a dream come true, but then he notices their knives... and guns... What kind of school did these girls come from?

Wild's High has a 42-year history as a fighting specialized, girls only, private high school meant solely for the elite. It is also the place that Wild's-League is held: the most popular event in the country and the only place in the world where teenage girls have brutal fights with their lives on the line.

One year the school changed from a girls only school to a mixed gender school. The only male student, Song Jae Gu, has been granted a full 3-year scholarship regardless of grades. Being someone who had to spend every moment either working or taking care of his younger brother and sister, he accepted.

But after meeting Queen, the bloody champion of Wild's-League, drenching her in coffee, calling her a monster, and getting caught seeing her while she was taking a shower... his scholarship could prove fatal.