Sexual assault is both a crime and a civil wrong. The district attorney decides whether to pursue a criminal action or not, you get to him by reporting the crime to the police in a timely manner, so they can gather evidence.
For a civil wrong, you are entitled to recover damages, the same as if someone committed an assault and battery on you and broke your jaw. Medical expenses, scarring, emotional distress. For a minor, sometimes their parents can be responsible for the damages.
There was a case many many years ago where 4 minors attacked a man in a park, and very seriously injured him. The man was able to recover from the homeowner insurance companies and other insurance policies held by the parents, for failing to supervise.
THE OPINIONS STATED HEREIN ARE BASED ON THE INFORMATION CONTAINED IN THE QUESTION,WITHOUT ANY INDEPENDENT INVESTIGATION. DO NOT RELY ON FREE LEGAL ADVICE IN A PUBLIC FORUM, EVEN MINE. CONTACT AN ATTORNEY OF YOUR CHOOSING IN YOUR LOCAL AREA TO PROTECT YOUR IMPORTANT PERSONAL RIGHTS.