In the following video, a Florida teacher punches a student in the face. The student crowded her into a closed door and was screaming vulgarities at her. The teacher threw two punches one landing square on the student's jaw. The student then retreats away and the video ends. In this case, the teacher was originally charged, but later the charges were dropped due to the video showing this may be a clear example of personal-defense to the State's Attorney.
I, for one, commend the teacher for doing what's necessary to defend herself, and for throwing a pretty technical right cross. I do not know if she has had any training, but I would not be surprised if she had. The teacher appears to give the student fair warning and indicates that his behavior is threatening to her. That is not always required, but if the situation dictates, I say again--if the situation dictates--it is a good idea to give verbal commands.
Verbal commands can bring attention to your plight and corroborate any testimony you give to the fact that you were not the original aggressor. However, if the situation is too dangerous or dynamic for such a thing, obviously verbal comments or warnings are not needed. Either way, defend yourself out there.