Yotsuba University Affiliated Junior High School

Yotsuba University Affiliated Junior High School (四葉大学付属中学校, Yotsuba-dai Gaku Fuzoku Chūgakkō) is the middle school in Onimai where Mahiro Oyama was enrolled when Mihari decides to have her go back to middle school as a girl. All of Mahiro's friends that are her age also attend the school. Mahiro previously attended the school as a boy several years ago, with Mihari Oyama and Kaede Hozuki having previously been students as well.

Faculty

 * Unnamed Principal
 * Unnamed Vice-Principal
 * Mari Nishinomiya (Homeroom Teacher)
 * Class 2-B (current)
 * Class 1-B (former)
 * Rin Minakata (Health and Physical Education Teacher)
 * Yuriko Kitahara (Home Economics Teacher)

Female Students

 * Mahiro Oyama
 * Momiji Hozuki
 * Asahi Ōka
 * Miyo Murosaki
 * Nayuta Tenkawa
 * Nemu Fujimi
 * Satsuki Tadaka
 * Minori Shiina
 * Mai Yasaka
 * Miyako Nagase
 * Nodoka Fushimi
 * Aoi Tachikawa
 * Akira Nakai
 * Misao Nogata
 * Mihari Oyama (former)
 * Kaede Hozuki (former)

Male Students
Note: There are also numerous other students that appear as unnamed background characters in both the manga and the anime.
 * Yuta Sakurada
 * Minato Senkawa
 * Takeshi Haijima
 * Tomoya Kiba
 * Makoto Kodaira
 * Mahiro Oyama (former)

Overview
Yotsuba University Affiliated Junior High School is a middle school in Oshimai. It appears to be close enough to where Mahiro, Mihari, Kaede, Momiji, Asahi, Miyo, and Nayuta live that they can walk there every single day. It is also close enough to the high school that Kaede attends that she can walk with Momiji on her way there and back on a daily basis.

It's name suggests that it is affiliated with a nearby university called Yotsuba University, though this place is yet to be seen.

There are seven horror legends associated with the school. One of these is a ghost girl who wanders the school after hours when no one is there.

Front Gates
In front of the school is a large wall with a single gate. This is how students enter and exit the school grounds every day.

Class 1-B
This is the class that Mahiro was enrolled in when she started attending classes as a girl. All of her friends including Momiji Hozuki, Asahi Ōka, and Miyo Murosaki were also a part of this class, having been in it since the start of the school year. Mahiro and her friends no longer attend this class since they became second year students in "Mahiro's Big Secret (Part 1)".

Class 2-B
This is the class that Mahiro and her friends are currently a part of. Every named character from Class 1-A was transferred into this class when they became second year students. Nayuta Tenkawa was enrolled in this class before the term started, and she quickly joined Mahiro's group of friends here.

Track
Outside the school is a large track for running. When Mahiro started gym class, the girls were in the middle of running laps for the month. Mahiro was able to sit out at least for the first day when Miyo mistakenly thought Mahiro was on her period and informed Rin.

Swimming Pool
Outside the school is a large pool used for swimming. During Mahiro's second year, Rin scheduled classes here with the boys on one side of the pool and girls on the other side. It was here that Mahiro's "comrade" regrew for the third time due to the medicine wearing off. Using the period excuse again, and with help from Nayuta, Mahiro was able to make it out of the pool without anyone finding out her secret.

Home Economics Classroom
There is a classroom with fully stocked kitchens used by Yuriko Kitahara for teaching students cooking and home economics. Mahiro and her friends baked black tea cookies here for a class assignment during their first year.

Auditorium
In the school, there is a large auditorium which can house the entire student body. Mahiro and her classmates attended the year-opening assembly here, hosted by the principal, when they began their second year.

Nurse's Office
There is a nurse's office in the school. Nayuta reports here regularly and has easy access to health supplies as Class 2-B's health officer. She and Mahiro came here after Mahiro wet herself during the year-opening assembly so they could get Mahiro fresh underwear.

History
Mahiro Oyama originally attended the school as a boy before graduating several years ago.

A few years later, his younger sister Mihari began attending and befriended a student in her class named Kaede Hozuki. Both girls had Mari Nishinomiya as their homeroom teacher, who was a new faculty member at the time. While attending, Mihari was an absolute prodigy in everything she did. She excelled at sports and set numerous records that remain unbroken to this day. She also regularly ended up at the top of her class throughout her entire time here. Kaede was busted by Mari for wearing makeup at least once. Eventually both girls graduated.

After Mihari set her "Onii-chan Rehabilitation Project" into effect, she decided that it was in Mahiro's best interest to attend middle school again, this time as a girl. She was able to dummy up some identification documents and subsequently made up a cover story that said Mahiro was ill and needed to be homeschooled to account for her previous absence. Once this was done, she secretly used these things to enroll Mahiro as a girl in Class 1-B.

Trivia

 * SchoolName.png name of the school is never mentioned in full in the manga. In the anime during "Mahiro Goes Back to Middle School", the full name of the school can be seen during a shot of the front gates.