Any school -- public or private -- in the United States has some responsibilities when it comes to dealing with reports of sexual violence or harassment on it's campus. Those responsibilities are listed under a Federal law known as Title IX. Title IX is part of the Education Amendments of 1972. It's worth noting that the law also covers reports of incidents that took place at school-sponsored activities and programs, not only on the physical campus of the school.
One of the first responsibilities of a school is to respond to a claim of sexual violence or harassment. The administration must do so in a timely manner and the response must be appropriate and effective. The school should eliminate harassment or violence and put measures in place to ensure that particularly harassment or violence doesn't repeat. The school is also responsible for providing some management regarding the effects of the harassment or violence.
The school's responsibility does not diminish simply because the student or parent decides not to file a complaint. If the parent or adult student doesn't formerly request no action be taken, and the school knows that harassment or sexual violence occurred, then the school is responsible for taking action. This is true even if the police have been involved and there is a criminal matter open. While a school must certainly comply with law enforcement during certain parts of an investigation, it is also required to conduct it's own internal investigation and take internal action regarding a report.
If you believe you are the victim of sexual harassment in a school, or that your child is, you should report the incident to the appropriate school administration and authorities. If you don't believe a school is acting on it's Title IX responsibilities, then consider talking to an attorney to find out what options you have to ensure the safety of yourself and your child and to seek any possible recovery for damages.
Source: U. S. Department of Education, "Know Your Rights," accessed March 08, 2016