The Independent School League or ISL is a group of seventeen preparatory schools located in the Washington metropolitan area of the United States that compete with each other athletically. All competitors in the ISL are girls, but not all schools are "all-girls" schools. The league is broken into two divisions: A and AA. The schools which compete in each division vary by sport.
References
Independent School League (Washington, D.C. area) Wikipedia(Text) CC BY-SA