So as a principal and parent I would do what you have already done. Unfortunately, as a principal you can't just ship out all the bad ones, only the really bad. Let the principal handle the investigation and decide what is appropriate given the situation and school laws/rules. Unfortunately as principal I am more of an investigative reporter trying to find the truth, intent and reasons behind an issue. I am not saying your daughter is lying or did anything wrong at all, regardless hitting is not OK. On the flip side when I go to a parent with a punishment, the more I know the better. Simply put, especially in middle school students, the first story you hear is not always the truth.
I would also contact the police, but only to file a complaint/report. Let the police go over and inform the other parents about the issue.
Do not over react, let the system work, but also do not let it be... The squeaking wheel gets the grease.
Hopefully it will not get worse.
I would also ask, where was your daughter when this happened? where was the supervising adult? Why did your daughter not report to teacher right away? These are all important questions. Granted all teachers and staff can not be everywhere at all times or see everything, but they should make every effort to do so.