First, she should have obtained a court order *requiring* her to be armed in public. These are not as uncommon as you might imagine, and trump local laws. Once she had obtained a local order, then she would need to present that order to a federal judge, so that it could be extended to also supercede federal laws against carrying weapons on school grounds.
Once this had been done, the school would have had no choice but to have permitted her to be armed on school grounds.
I highly recommend that if you need a concealed weapon for immediate personal protection, that is, you have a reason to need it right now, you get such a legal order. Otherwise, there are any number of hassles you will get from all levels of government and business.
This is interesting. I've never heard of this.
Do you have any links to more information about this?