Ah! Itoshi No Banchousama summary:

Hirayama Hayaka’s parents got divorced, and now Hayaka and her mother are looking for a fresh start in a new town. To make life easier for her mother, Hayaka decides to abandon her private school days and enrolls in an industrial high school without touring it. On the first day, after witnessing a fight between her class’s leader and an upperclassmen, Hayaka learns that all the girls in the school transferred out months ago, because the boys were really out of hand and always fighting. Hayaka is the only girl in school!The next day when Hayaka comes to school, the same class leader and his friends adopt Hayaka, and she learns that not all the gansters in school are bad! The class leader turns out to be a caring boy, who mothers all the boys in the class, and then extends his care to Hayaka.As the class leader knows that Hayaka is in danger if she stays unprotected on school grounds with so many gansters around, he puts a metal plate in her book-bag just to be safe. But when a fight ensues between the school gang leader (the banchou) and the class leader, Hayaka hits the banchou on the head with her bag, knocking him out!As school law among the students states that whoever defeats the banchou becomes the new banchou, Hayaka is the new school gang leader! As the story goes on, Hayaka gains more and more subordinates in different ways. She meets other school gang leaders and rich heirs while increasing her number of subordinates, all while unknowingly charming the class leader with her purity and determination to do the right thing…