In my experience as a teacher, it seems rather clear that Debbie is a bully who takes delight in bullying other children who are not as dominant. You were definitely right in contacting the school regarding the incident on the bus. They can only act if they know about the problem. While the action they take the first time may not be as strong as one might want, chances are Debbie will do this again, probably in the near future.Once they are alert to the problem they are in a better position to intervene