Problem is the law is against adults and teachers when it comes to children.
Yes children need to be protected, but if a student comes in and pulls a knife out, you should be protected from the law if you intervene.
If it happened to me, I'd be far more scared of the consequences of the law if say I intervened and said child was seriously injured or died of my actions, than getting stabbed.
A the little ***** know this, not only that but because the law the law works, they know they'll more or less end up with nothing more than a slapped wrist.
What needs to happen, is some kid does this, teacher lamps them over a head with say a fire extinguisher or whatever is at hand, and that kid dues, and they are protected from the law, ...next kid might think twice.