[Sonar5]

I'm not sure what the rules are, or if they require my son to do something.

When the three went down there, the one who brought it was sitting there with his head down, menaing two things.

1) Some other child told an adult.
2) They probably got the knife and the kid confessed.

Why subject the other three to this treatment if it was already being addressed?

[HairOfTheDog]

Because they were with the kid when he had the knife, and they knew and saw and probably all held the knife. They want the kids to know in no uncertain terms knives aren't allowed. Why are you interfering with that lesson?

[MagnumRancid]

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Why subject the other three to this treatment if it was already being addressed?
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You just don't understand the "Bureaucratic Process". You are dealing with government employees. Their primary objective is to cover their fannies. There could be numerous secondary objectives.
Some lonely, bored administrator might not have enough drama in his personal life.
It might be a chance to check out the new "Student Offender Tracking System".
It's the government, it doesn't have to make sense.