Ah! Itoshi no Banchousama

From CMX:

Souka and her recently divorced mother move to a new place to start over. In looking for a school to enroll in, Souka decides to leave her preppy, private high school behind and transfers into a technical high school. To her surprise, she's the only girl student in the entire school! The first day of school is nothing like she ever imagined -- boys crashing through the window, fighting for all they are worth. One day, the school's current "Bancho" (a term for a gang leader) ambushes Souka. Determined to protect her, Yu, one of the classmates, comes to her rescue, but Yu ends up in a struggle. Trying to help, Souka swings her book bag and ends up taking down the Bancho herself! What Souka didn't know was that when someone takes down the class' leader, you became the leader of that class. This wasn't a role she was expecting for herself, but will she be able to relinquish it?

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From Esthétique:

Due to family problems, Hirayama Souka tranfers from a private high school to an industrial high school. However, upon entering the school, she finds out that there are 850 boys in the school, and that she is the only girl there. Apart from the great disparity between the number of boys and girls, she also finds out that gangsters still exist in this era. Furthermore, a conflict has erupted between her classmate Katou Yuuni and the gangsters. Thus begins Souka's eventful life in the industrial high school...