It shouldn't really be an issue, then.
I am not sure how the Marines found out about it, unless there was a school record of the expulsion and THAT was what caused the problem. A law enforcement background generally is not going to pull your middle school or even high school records beyond your transcripts. Even then, the fact that a 12 year old passed off his prescribed ADHD pill to a classmate is not going to raise serious concerns by itself. Yes, you may have to admit to this event, but, I cannot imagine it will be an issue. It would not be one out here in CA. You might consider talking to a background investigator at a nearby law enforcement agency in your part of FL to get their impression on it.

