First off, the school has an obligation to keep that woman away from your son. How she got into the position to be in the school and threaten him is something I would like to know.
You have taken the right steps by notifying the school that she was trying to threaten and intimidate you son on their watch and by your notifying the police. Now you have a paper trail. If anything else happens, you have her in a very damaging position.
Stay in touch with school officials and see what followup has been taken. If they don't protect your son, they are a position to be held liable.
