Ah! Itoshi No Banchousama

Because of her parents divorce, Hirayama Hayaka is forced out of a private school into a industrial school. When she arrives, she finds out that she is the only female student in that school, because all of the other girls transferred out! Apart from the great disparity between the number of boys and girls, she finds out that gansters still exist in this part of town. A conflict had also started between her classmate Katou Yuuni, a motherlike boy, and the gangsters. One incident leads to another, and she hits the previous school gang leader (and accidently knocks him out)! Furthermore, whoever defeats the school gang leader will be crowned as the new one. What does that possibly mean? Hayaka is now the school gang leader?!
New Summary from the CMX website
Souka and her mother are looking for a fresh start, so they move to a new place where no one knows them. Souka embraces the idea of starting over and leaves her private school days behind to enroll in the local tech academy. But the first day of school is nothing like she imagined it would be—she is the only girl around! Unfortunately, not everyone welcomes Souka with open arms, including the school leader. But when she takes him down in front of everyone, Souka becomes the new boss on campus!