Whether the principal or superintendent can "stand up" for the student depends on the circumstances at the moment the knife was found. For instance, if the principal found the knife and was alone at the time he/she could have not reported it (unlikely, but possible). As soon as somebody else knows about the knife - it's over. The policy is a School Board policy and the Board has to stand by the policy or get skewered by the media. Public entities are stuck when it comes to this. That's why, whenever possible, they don't have policies but rather something they might call guidelines or procedures - they have leeway on those. At least that's how the large Govt organization I work for handles things.

When my son was in 5th grade a classmate heard him say something like "Oh, yeah ... well if he did that I'd kill him." The classmate told the bus driver who told the principal who apologized to my wife and I for suspending our son. The principal said it was all about zero-tolerance and the Board of Education gave all administrators very clear instructions - personally he thought it was stupid and implied that it was insulting to him that he had no way to use his discretion on so may things.