I think you’ve somewhat missed my point, Zeuhl.
I’m not suggesting that (whatever the chronology) if the attempted poisoners had been treated more seriously the Leeds teacher stabbing would have been avoided. It certainly would have made no difference to that. However, your question “… why anyone would want to expend energy and resources on things that make no difference.” is a little strange.
Treating the ten year olds as criminals might, just might, have made them realise that poisoning people is a serious matter and they might, just might, grow up with a little more sense and consideration. It might also serve “pour discourager les autres”. But then none of this might happen. However for the police and the CPS to allow the school to handle what seems essentially a serious crime on the basis that taking proper action “will make no difference” is very curious. Presumably, given the irrational nature of the fifteen year old who killed his teacher any punishment he might receive is unlikely to make any difference. So are you suggesting we just ignore the matter? Or perhaps we should let the school (whose wish is probably just to “draw a line under the issue and move on”) deal with it “appropriately”.
My point is that if you allow ten year olds to pour bleach into their teacher’s drink without proper sanction you should not be too surprised if, by age 15, they feel ready to put a knife in her back.