Should I mention a disciplinary action in the application process when applying to a university?

There's no real way for me to full state what I want to in the title box, so here it goes.

I am 24 now, when I was 16 I was expelled from my high school. I still graduated on time from a different school, but in transferring from my community college to a university I'm not sure how to deal with the subject.

The background: I was expelled because this one girl I was friends with in high school called the sceretary of our school from my home phone and gave a death threat. I turned her in the next day and was still expelled, however some leanancy was given to me, I believe simply because it was clear I was guilty by association, its not on my record. I have offical copies of the high school I graduated from as well as the one I was expelled from. There's nothing about an expellision or anything.

The transcripts have a box there called Reason for Leaving, under it one says "Graduated" and the other simply says "Public in Dist." It also states me leaving that school when I would have graduated, not when I was expelled. I have searched every box and I can't find one thing in there that says anything about it. Here are the transcripts (the only info that is marked out are ones which somehow identify me: http://imgur.com/izJPF,mjwFW,sN6FC#0

Here is my explanation to the school that I am applying to under "Have you ever been suspended or expelled from any school":

"When I was 16 and in high school, a friend I had at the time choose to make a prank phone call from my house that I did not condone. She threatened the secretary using the phone from my parent's house. I kicked her out and the next day I turned her in. However, I was suspended for the rest of the year despite my not having any involvement other than that person using my phone. I was still able to graduate on time by attending another school.

I learned a very big lesson from this event. I learned the importance of the friends you have and how the things we do affect our opportunities the future may hold for us.

Since then, I have managed an impressive GPA in college, an internship with the Democratic National Convention Committee in the department of Communications and Public Affairs, joined a LGBTQ group called SPECTRUM at Central Piedmont Community College, and even joined Phi Theta Kappa Honor Society.

Please, let this mishap in no way affect your decision."

Thanks so much for the insight! I want to be as mature about this as possible. I am even considering simply making an appointment with admissions and just seeing if I can explain it face to face.