While I have not looked up the crime in your state,generally such threats are criminal even if made to a second party.
As a note, to be a very serious crime, this threat would likely have to credible. Simply writing an angry note saying "I'm going to kill her!" is not likely to result in any criminal action. It could, however, result in action by the school, and may even be considered a lesser offense for threatening the teacher, but absent some other credible proof that the threat might be serious I doubt this will be too big a crime.
You would be best off if you consulted an attorney.
Do you know the code section (that should be a series of numbers) that she has been charged with violating?
