Per AMCAS application under items that you agreed to was the following provision
I understand that I am required to inform the Admission Office of each medical school to which I apply if I am convicted of, or plead guilty or no contest to, a misdemeanor or felony crime after the date of my original application submission and prior to medical school matriculation. I understand that this communication must be in writing and must occur within ten (10) business days of the occurrence of the conviction
So if you are in a court-sponsored program with charges pending, and then dismissed, you are not in technical violation of the provision.
However, if you pled anything that received a "suspended sentence" or similar with the agreement the plea would be vacated upon completion, then you are in direct violation of the provision. In either case, the dean can rescind the acceptance offer and it is unlikely you would win in court on this.
At this point, and with the little information I have, I presume you have not had a criminal background check done yet. This will may show up and you may need to explain it in a letter to the dean. I am not sure if you come forward now proactively would it be better or worse.