I suppose I'd say that if this is all there is to the story, the principal is completely wrong. But many times, lawyers will advise their client to say nothing, so you really don't get the full story until the actual trial/hearing.
From the principal's comments, he really appears to be saying that its a zero tolerance policy, and that's the end of the matter. I think he's pretty clearly wrong on that, but we'll have to see how it comes out.
The principal may be reluctant to create an "I forgot" precedent that will let students who bring weapons to school intentionally claim they just forgot about it later if it's discovered. I still think that's crummy rationale, because there's no harm in letting a student turn in a knife or something before it is discovered rather than after.
Anyway, I don't blame you for your skepticism. We've all been burned too many times by one-sided stories that turn out not to be completely true.
The crux of the matter, as you indicate in your last sentence, is that the student proactively did something to avoid continuing to break the rules. In recognizing that having the knife was against the rules, and then by trying to turn it in, the boy did right (I'm not talking about the rule itself, BTW).
No good deed...