First of all, it wasn’t a knife. It was wood in the shape of a knife.
Next thing is to suspend anyone who carves wood or soap in the shape of a knife and accuse them of posessing a knife.
Only completely stupid idiots can’t tell the difference between a thing and a representation of the thing.
Would you be saying the same thing, if he had then taken the 'knife' to Art class, and crafted a wax 'representation' of his math teacher, then killed him by sticking the 'knife' into the voodoo doll of the 'teacher'?
Schools must consider all scenarios, for every one's safety. It's for the children!
/sarc.