Hi!
I use a small penknife/craft knife to sharpen my pencils.I use it because I can get a much better sharpened point than I can ever get with a poxy ordinary sharpener.Mind I am an art teacher so I can claim that it is part of the tools of my trade.I NEVER let students use it and I never leave it laying around and I NEVER threaten anyone with it and I never use it anywhere but over a bin.As soon as I've finished it gets put away in my personal art pencil case in my bag.
Anyone goes in my personal bag they get put up in front of their head of year and the head.
I think someone is over reacting here.If you have NEVER been threatened by him why are you so worried??
You sure it is 4 inches long?I thought today's modern youth were measuring in metric..
You really shouldn't judge staff by the standards that maybe you do for any of your school friends.We can be trusted and I refuse to be treated like a little child by those who are children.....
It's so like that perennial suggestion that goes like this. " I think that if I have to wear a uniform in school so should the teachers" Wrong! I spent 14 years in a school uniform(including sixth form) and I don't see why I should as an adult.
Anyway as far as I know I AM AN ADULT and as long as I threaten no one I can carry a penknife as a tool.I very much doubt of the head teacher will do anything but give you a pained look....I also suspect that the police will also give you a funny look....Plus your suggestion of an anonymous letter shows you already know the reaction you will get.
Of course IF HE HAS THREATENED ANYONE then you must act by talking to your parents and getting them to sort it out.... That would be the only way to approach it...