Misdemeanors, expungement, and ROTC ?

I'm in college and interested in joining ROTC but I'm confused on how to answer if they ask me if I've ever been arrested. When I was in high school I was arrested and charged with a misdemeanor (marijuana) but the charges were dropped. I'm trying to get the incident expunged from my record so I can deny it ever happened but will that do any good or should I not bother getting it expunged and explain the situation? Also is my dream of joining ROTC gone forever because of this one mistake or is there any hope for me?