From Wikipedia:
After School (Korean: 애프터스쿨) is a South Korean girl group formed by Pledis Entertainment in 2009. They were known as the first group to debut with a unique admissions and graduation concept in Korea, where members are added or withdrawn in a natural manner. The band consists of eight members: Jungah, Jooyeon, Uee, Raina, Nana, Lizzy, E-Young and Kaeun. Graduate member Soyoung graduated from the group in fall 2009, Bekah officially graduated from the group in July 2011, and Kahi graduated in September 2012.