To determine whether behaviour is problematic or not you need to gather information about the event and discuss with colleagues and management, but most importantly: discuss with professionals. The Traffic Light is also a good tool to use in the assessment.
Make sure the school looks after all the children and persons affected throughout this investigation phase. Create an environment in which the pupil can recount their experiences, and opportunities to be closer to the pupil. Inform the pupil of what is going on.