I think that the best thing to do is to redo it, as well as writing a note to your teacher. Because the essay wasn't done, it would just be better to write another one, but make sure that the new essay is a lot better than your old one. Make sure that when writing your note to your teacher, you tell him/her that you accidentally left it at school, but it wasn't completely done. You should email them, and because you might have been half-way done with your essay, they may give you extra time to do it. However, they might just make you create a whole new one, so just do both A and B.
~Hope I helped, and good luck on your essay!~