I'm glad the police officers handled that so professional, but being expelled, because A) for a formal reason, as you didn't lie (Nice adult world we are living in right?) and B) for that rather minor offence (it was an offence, but come on).
I believe, judging from your story, they tried to create an example. ("Don't bring ANY kind of weapon to school.") In other schools pupils beat up, threaten, extort and blackmail other pupils and even though they get caught, they just have to write an essay about their misdeeds or are suspended for a week or something. And afterwards they happily continue. Such persons should be expelled (for good). An essay and/or a suspension for a few days would have been more then enough in your and your friend's case. I mean, the story with the police was shockingly enough for you, as you remember it for your whole life. Instead they choose to hurt your school career for fooling around like a kid/teen/(even young adult) should. That was unreasonable harsh.