A sandwich with butter

At school one of the teachers told me that one of my sons was teased without any provocation from my son. She sat next to him and suddenly a boy come by and that boy smeared a sandwich with butter on the face of my son. My son became very sad and the teacher next to him was surprised this could happen while she was sitting next to my son. She could comfort my son and when I came my son was already gone to his play date. It makes me worried because now the teacher saw what happened and it is not the first time.