It may be helpful to talk about the experience with friends, a counselor or family as well. You are probably not alone in considering the harassing behavior objectionable. Follow grievance procedures outlined in your campus student handbook. Talk to your supervisor, the harasser's supervisor, your teacher and any fellow student you think might be helpful. Make a report.
Every campus has a sexual harassment policy.
Often sexual harassment complaints can be resolved informally, but the policy should also provide procedures if you wish to pursue the matter more formally. You deserve to feel safe and comfortable while serving the community. A sexual harasser is not only taking from you, but also jeopardizing the services you provide as a volunteer.