It depends on what kind of story you're looking to write... and on the teacher. I suggest something with suspense, detective work, anything that will get the reader's attention and keep him on the edge. Throw in a killer or suspicious character, you know, an element of surprise
The point is not to write the same stuff as the rest of your mates. If you teacher is as strict as mine then you need to stand out (and have perfect grammar and punctuation too...) I've tried to write about anything from poetry to horror stories and my teacher still didn't give me an A, so it's partly about the luck too...
Oh, and if your teacher is one of those people that thinks you just ripped off someone and didn't do any work yourself, then keep all your drafts. It happened to one of my mates, the teacher just ripped the essay in half in front of her... >_<