But if a school employee, permitted to be armed on school property, shoots somebody (or while trying to nail an attacker shoots a bystander), then that IS something the school can be sued for.
This is fairly easily dealt with. Modify the law so that any business that refuses to allow people to protect themselves would fall under a very strict liability, whereas anyone protecting himself or others would be covered under 'Good Samaritan' acts. Problem solved! The insurance companies would be falling all over each other to insure.
That IMHO should be the proper way to handle the question of whether businesses allow employees to carry weapons onto company property. If companies have a really good reason not to allow employees to carry firearms to protect themselves (e.g. because metallic objects could be hazardous near large magnets) then they need to be responsible for protecting their employees, from the time they leave the last place they can store their weapons to the time they return there.