can I sue teacher and school district

I need some advice on a situation. My nephew has been sexually assaulted, statutory rape, personal and emotional injury by his teacher. The first incidence happened in Oct 2017 and three police were involved yet the school failed to involve his guardian. She had no knowledge of the situation. The police said nothing came of it so they did not notify her. It was a situation involving a minor. The guardian should have been notified. The teacher has been texting my nephew threatening him. They now have proof and have started an investigation. What can we do to begin legal action. My nephew is having a very hard emotional breakdown about this. It's a very small town. He does not even want to finish high school now.