Such a letter seems to be more powerful than a verbal request. Report sexual harassment to the appropriate person in the organization. Schools and employers are required to have policies and procedures for dealing with sexual harassment. You can use those policies and procedures to make an informal or a formal complaint against the person who harasses you. File a complaint with your state agency that deals with employment discrimination .
Many states have laws against employment discrimination and sexual harassment.
Your first legal step should be to file a complaint with the appropriate organization in your state . The research librarian at your public or university library can help you find information on state laws and how to file claims. The Equal Employment Opportunity Commission is a federal agency that enforces federal anti-discrimination laws. Some states automatically notify the EEOC if you file a complaint with your state agency, or you can file with both. There are some quite strict statutes of limitations for filing claims.