You may have the same level of difficulty as I have accepting that the "guilty dog look" is a myth. Look at that little face in the photo below - it makes me want to pick him up and love him so he doesn't feel so bad.
According to recent studies dogs do not feel guilt. It has been determined that the look we are mistaking for guilt is actually fear and that the dog is simply reacting to our reaction. Further to this, they have concluded that we are projecting our human emotions on to the dog.
Think of the difference between your body language during your lecture and during an average daily greeting or interaction. You are using a different tone of voice, moving erratically, giving very stern looks, and not touching your dog as you normally do. Your dog looks at you and sees a very different individual than he sees on an average day. ~ Dog Speak: Dogs and the Guilty Look