Irrespective of the law, a teacher should not be brandishing a knife in front of impressionable students, certainly not when there are cheaper, safer viable alternatives (20p pencil sharpener).
If the blade is 4 inches then that's clearly illegal.
Does the school have a policy that states "no knives" ?
perhaps if it has, then you could bring this up next time he gets it out & say, "sir, if the school has a policy that say no knives, then how come you are allowed to have one, sureely that's against the rules" sort of thing?
That would open the discussion & you could also drop the law into it, then ask if he thinks it's a good thing that he uses one in front of all his students etc?
At the end of the discussion., when he is obviously winning by virtue of him being "sir" & making the rules, offer him this shiney new pencil sharpener that youhave bought if he put's it away.
He might then think twice about it, or he may just ignore you.
Our resident teaching expert will advice you about what is & is not allowed in schools regarding the matter....Coggy, where are you?