Get one 5 inches long?
Depends what they are carrying it for - but I thought it was now illegal to be carrying any sort of knife in public anyway? I still have my Swiss Army Knife in my handbag though.
I think it seems a bit strange for a teacher to have a knife about them, although the CDT teacher at a school I used to work at seemed to keep a full tool kit in his pockets (including a couple of knives).