She belongs to the same class as Chiyo Kurihara and Kiyoshi Fujino, and is a member of the school's Go Club.

The friendly character of Mayumi Tanaka and her good friendship with Chiyo is shown when she expresses concern about Chiyo when gloomy. While Chiyo relies on her for advice and mental support as a good confidante, Mayumi is a non-confrontational person and tries to avoid fights - thus inadvertently depending on Chiyo for physical defense and/or protection. Though she does not seems antagonistic towards the boys, Mayumi is apathetic towards Chiyo's interest in Kiyoshi & the enrolment of the five first-year boys in the school. In contrast to Chiyo's love of sumo wrestling, Mayumi shows an interest in clothes and magazines. However, Chiyo is completely oblivious to her disinterest in sumo wrestling.

(Source: Prison School Wiki)

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?!. 
 
(Source: Anime News Network)

On the outskirts of Tokyo stands Hachimitsu Private Academy, an all-girls boarding school for elite young women. But as the new school year rolls around, one tradition is going out the window: For the first time in the school's long history, boys are being allowed to enroll. On the first day of school, though, it turns out only five boys have made the cut. Thrilled by this discovery, these young men's adolescent hearts leap with joy at the thought of being surrounded by all those girls, but little do the five boys suspect the shocking fate that awaits them... 
(Source: Yen Press) 
Notes: 
- Includes 1 extra chapter and the epilogue. 
- Won the Kodansha Manga Award 2013 for best general manga.