What they did was to interrogate a child not even suspected of wrongdoing.
I think you have it wrong--the administrator was building a paper trail for the probable expulsion hearing of the boy who brought the knife. And this will most likely be put in an event file not in the cum record.
The police "interrogate" witnesses all the time who are not suspected of wrong doing.
As I said, if they had suspended the boy or charged him criminally with something I could see a reason to be upset. But that didn't happen.
When it's all said and done, he had to take home a note. That's it.
Are you telling me when you were a kid that no one was ever sent to the principal's office without a parent present?