Haha, had to laugh at how the boy thing was handled in like two panels. "Yeah, somehow boys exists in this universe, but you wont see anything but girls from now on"
When I was around that age, I also had 2 dance courses (standard dances) and sometimes there was a girl surplus too. But not as much as in this manga. ;P
Anyways: my guess. Shion (the ex) either was really annoyed by her, or what I rather think after chapter 0.5. She fell in love with Kiki and couldnt handle being near her all the time. Maybe also that she couldnt convince her being herself, in this case accepting the follower role.
Lets see if there will be a triangle.