Do not allow your children to go to parties or sleepovers unless there is an outside chaperone (such as YOURSELF) as a monitor. DO NOT TRUST any parents from your child's PUBLIC SCHOOL. DO NOT TRUST church parents, youth ministers, priests, coaches, etc. -- offer to go along with your child, or else YOUR CHILD DOES NOT GO!!!
Better to hamper their social lives than have them subjected to sexual and mental abuse!
FWIW, this article says that the girls all attended the same DANCE school.