A) Instruct your children - They DO not and WILL NOT “talk” with the police without your being there. Period. Inform the school of same - in writing, keep a copy.
B) Tell the Administrator that you will “seek council” to be “made whole” due the trauma your child suffered due entirely to their “proven incompetence”.
C) Insist the real author of the note be expelled immediately for making “terroristic threats” as required by MISSISSIPPI SCHOOL SAFETY ACT OF 2001. (Interesting law BTW)
D) Strongly consider pulling your child and insist the District provide an in home tutor until YOU are certain the threat to your child has been resolved.
or
Best outcome) Home school you child.
They are obligated to investigate anything that is suspicious. Sounds like you should be able to clear it up with no problem.
Utterly inexcusable. You need to go to the school board and superintendent. And take your lawyer, if necessary.