I agree with Bones. If you've sent School A a LOI, but have already been accepted by School B, you need to tell School A. Schools aren't interested in making sure *you* get into med school, rather, they want to make sure they have the best students. They're likely to look at your app again to see why School B accepted you. Plus, schools want students who want to be there, so if you tell them that even though School B has already accepted you but you'd still rather attend School A and would drop School B if you were accepted, then it could possibly help you.