If you treat students like prisoners, they're going to act like prisoners.
True enough. But kids are treated like inmates far more by their predatory peers (gang members, drug dealers, pathological bullies, and other head cases) than they are by school staff.
Zero tolerance for 'predatory peers' is a good idea. As a substitute I show no tolerance for bullying. Head cases? I have Asperger's, which by some definitions could make me a 'head case'.
Many kids have no choice about being 'head cases', and schools must accommodate kids with handicaps.
Teaching and law enforcement are incompatible. A teacher with a gun would lose trust quickly, and teachers would be in deep trouble if they forgot to guard a gun while they instinctively responded to a questionable situation.
My sympathy to your child. Some schools have placed cameras in classrooms, and those are automatically on in K-12 classes in which the school concentrates trouble-makers and on if the teacher or an administrator so desires. I would turn one on in the event of a medical emergency.
It may be appropriate to have a second teacher (arguably an enforcer or deterrent) following the trouble-makers around, especially if there is a substitute or a new teacher.
In my case I have gotten warnings from the regular teacher at the class in which I substitute, and I have no qualms about warning the student -- if with a little subtlety, as in "Bill, I expect you to be on your best behavior today because I don't enjoy writing referral notices". I love to write "NO DISCIPLINARY PROBLEMS TODAY!" on my report for my day's work.