From Chibi Manga:

Being raised in the middle of the mountains and while living in the countryside she was the only student in Elementary and Junior High School. And that Mikan transferred to a school in Tokyo! It’s the beginning of a school life that she’s always yearned for... it was supposed to be but, she was attacked by a group of male students. Mikan is the first female student to attend the school since the school has been founded. Although, Kai-kun, who is in the same class as her protects her, but everyday turns into a dangerous situation.