Bullying is a serious national health issue. Social science study of bullying traditionally has used personal surveys in schools, suffering from small sample size and low temporal resolution. We are developing novel machine learning models to study bullying. Our model aims to reconstruct a bullying event -- who the bullies, victims, and witnesses are, and what happened to them -- from publicly available social media posts. Our model and data can improve the scientific study, intervention and policy-making of bullying.