Declaring an interest: I am a teacher in an urban comprehensive British school.
I find it highly unlikely that a teacher would end up getting shot/knifed at school. Most violence by students against staff is not pre-meditated (and even then incredibly rare in most schools, though I do cede that there are a very few schools where it is not treated as the instant kick-out that it should be). I have known of one student bringing a knife to school, and apparently he needed it to enforce a drug contract after school - needless to say the school told him where to go. I have never known any student to carry weapons to school for their perceived own protection, or to wear stab vests.
We've definitely had a few hammers brought to school, or better yet, stolen from the woodwork workshops, though the intention there is to damage property as opposed to persons.
This is true at my school also.
