ScaredWebWarrior

First of all read this http://www.bushcraftuk.co.uk/articles/knife_law.html

There you will find that "A folding pocket knife is not included, so long as the cutting edge is under three inches."

Also, if there is a 'good reason' to carry such a knife (or even some that might otherwise considered off-limits) then again the law would be unlikely to be enforced.

I don't ordinarily put a knife in my pocket (in fact - I don't think I ever do) but if I'm going into the outdoors I will have an extremely sharp and effective knife with me. And not with any notion of offence or defence, but as a tool. But probably in my backpack/day sack.
Hence I can easily provide 'good reason' and it clearly is not in any way carried as an offensive weapon.

So it is not the knife as such, but the how/why of it being carried that is the issue.

If this teacher has used or displayed the knife for any reason that cannot be justified, then that knife irrespective of its size etc. becomes an offensive weapon under the law.